Preparation method of lithium magnesium silicate Technical Field The invention relates to the field of chemical synthesis, in particular to a preparation method of lithium magnesium silicate. Background Lithium magnesium silicate, CAS No.-90-9, molecular formula Li2Mg2O9Si3Belongs to bentonite montmorillonite, has a hexagonal crystal structure, is white powder in appearance, is nontoxic and tasteless, and is commonly used as a thickening agent, a suspending agent, an anti-settling agent, an adhesive, a thixotropic agent and a dispersing agent. The hydrated gel has excellent rheological property and is widely used in medicines, cosmetics, paints, pesticides, toys and the like. The synthesis method of the magnesium lithium silicate is mainly a hydrothermal synthesis method, and the magnesium lithium silicate is prepared by mixing soluble silicate and magnesium lithium oxide in proportion and reacting for a long time under the conditions of high temperature and high pressure, but the long-time conditions of high temperature and high pressure are easy to cause potential safety hazards, and the produced magnesium lithium silicate also has the problems of uneven chelation degree, too hard texture and unstable batch quality, so that a preparation method of the magnesium lithium silicate which has normal-pressure reaction and high safety and can be suitable for large-scale and large-scale production needs to be found. Chinese patent CNA discloses a magnesium lithium silicate compound and a preparation method thereof, which adopts a normal pressure reaction to avoid the potential safety hazard problem of long-time high temperature and high pressure conditions, but on one hand, because the preparation method does not adopt a high pressure condition, the conditions of normal pressure condition and other parameter control are not optimized, and on the other hand, on the selection of raw materials and their mixture ratio, the produced magnesium lithium silicate still has the problems of non-uniform chelating degree and unstable quality between batches. Therefore, there is a need for a magnesium lithium silicate which can be produced under normal pressure conditions, is highly safe, and can produce a lithium magnesium silicate having a uniform and stable crystal structure unit. Disclosure of Invention In view of the above problems, the present invention aims to provide a method for preparing lithium magnesium silicate, which can be conveniently produced under normal pressure conditions and can prepare lithium magnesium silicate with uniform and stable crystal structure units. In order to achieve the purpose, the invention provides the following technical scheme: a preparation method of lithium magnesium silicate comprises the following steps: (1) dissolving lithium chloride in water in a reaction kettle, heating to boil, and keeping boiling for 2-4 min; adding a magnesium sulfate solution, and keeping boiling for 18-22 min; (2) dropwise adding liquid water glass into the reaction kettle at a constant speed; the uniform dripping speed is as follows: 0.4 kg/min-0.6 kg/min; (3) dropwise adding a sodium carbonate solution into the reaction kettle at a constant speed; (4) after the dropwise adding is finished, keeping boiling, and reacting for 17-20 hours to obtain the product; wherein the molar ratio of silicon, lithium and magnesium is (2-4): 0.9-1.1): 1; the molar ratio of the sodium carbonate to the lithium chloride is (0.4-0.6): 1. The invention also provides a magnesium lithium silicate, and the specific technical scheme is as follows: the magnesium lithium silicate is prepared by the preparation method. Based on the technical scheme, the invention has the following beneficial effects: the inventor of the invention finds that the specific types of the raw materials, the adding modes and the adding sequences thereof and the performance of the synthesized magnesium lithium silicate are different, and the adoption of the proper raw materials, the proper proportion, the adding modes and the adding sequences thereof and the matching of pressure conditions is of great importance for the stable crystal structure formed by the magnesium lithium silicate. The invention adopts a hydrothermal synthesis method under normal pressure, water glass and sodium carbonate are dripped in a uniform speed dripping mode under the condition that liquid in a reaction kettle keeps boiling through a proper raw material adding sequence, and the dripping speed of the dripped water glass is controlled, so that silicon, magnesium and lithium oxygen bonds in magnesium lithium silicate can be formed at a proper and uniform speed, the magnesium lithium silicate forms a stable crystal structure, the stable crystal structure and a firm multi-molecular spatial structure are provided, and the effects of stable thickening, thixotropic property, suspension, anti-settling and other aqueous rheological additives are realized. The powder particles of the lithium magnesium silicate prepared by the preparation method are more uniform and fine, and are favorable for being compatible with other components in a formula. The gel dispersion liquid prepared by the invention has the advantages of high degree of separation of particles into single small pieces in a colloidal structure, firm structure, high viscosity, obvious thickening effect, high transparency of thixotropic gel formed by expansion, and high application value in the fields of multicolor coatings and the like. Moreover, the invention adopts mild raw materials, does not add any strong acid and strong alkali in the production process, reduces pollution emission and is beneficial to environmental protection. Detailed Description In order that the invention may be more readily understood, reference will now be made to the following more particular description of the invention, examples of which are set forth below. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete. Each of the raw materials used in the examples is a commercially available product unless otherwise specified. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. The embodiment of the invention provides a preparation method of lithium magnesium silicate, which comprises the following steps: (1) dissolving lithium chloride in water in a reaction kettle, heating to boil, and keeping boiling for 2-4 min; adding a magnesium sulfate solution, and keeping boiling for 18-22 min; preferably, in the lithium chloride and the magnesium sulfate, the molar ratio of lithium to magnesium is (0.9-1.1): 1; (2) dropwise adding liquid water glass into the reaction kettle at a constant speed; preferably, the uniform dropping speed is as follows: 0.4 kg/min-0.6 kg/min; (3) dropwise adding a sodium carbonate solution into the reaction kettle at a constant speed; (4) after the dropwise adding is finished, keeping boiling, and reacting for 17-20 hours to obtain the product; further preferably, the molar ratio of silicon, lithium and magnesium is (2-4): 0.9-1.1): 1; the molar ratio of the sodium carbonate to the lithium chloride is (0.4-0.6): 1. Wherein the crystal structure unit of magnesium lithium silicate is a tiny flake with the thickness of nanometer, and the surface of the flake is covered with exchangeable cations, wherein the exchangeable cations are mainly Na+. When the lithium magnesium silicate particles are mixed with water, the water is mixed with Na+Contact is made to the surface of the sheet to hold the gel open along the sheet, at which point the particles expand rapidly until the sheet separates. Because the surface of the slice is negatively charged and the end surface is positively charged, the separated slice end surface is attracted to the surface of another slice, thereby rapidly forming a colloid structure of three-dimensional space, namely a card palace structure, increasing the viscosity of the system, and having high suspension and thickening propertiesThixotropy, good compatibility and chemical stability, and forms an ideal water system thickening rheological agent. Therefore, the uniformity and stability of the crystal structure unit of the magnesium lithium silicate play an important role in the performance of forming colloid and colloid after the magnesium lithium silicate is dissolved in water. According to the invention, by adopting the proper raw materials and the proportion, through a proper raw material adding sequence, under the condition that liquid in a reaction kettle keeps boiling, water glass and sodium carbonate are dropwise added at a constant speed, and the dropwise adding speed of the water glass is controlled, so that silicon, magnesium and lithium oxygen bonds in magnesium lithium silicate can be formed at a proper and uniform speed under normal pressure reaction conditions, the magnesium lithium silicate forms a stable crystal structure, and the effect is finally realized. Water glass is a soluble alkali silicate material, also known as soda-lime, formed by combining alkali oxides and silica. The water glass can be divided into sodium water glass and potassium water glass according to the types of alkali metals, and the molecular formulas of the water glass and the potassium water glass are respectively Na2O·nSiO2And K2O·nSiO2The coefficient n in the formula is called a water glass modulus, and is a molecular ratio (or a molar ratio) of silica and an alkali metal oxide in water glass, and the water glass modulus has a correlation with a silica content and a water glass viscosity. In the invention, the modulus of the liquid water glass is preferably 3-4. The water glass modulus in the range is favorable for the prepared magnesium lithium silicate to have good viscosity. More preferably, the water glass is water-soluble sodium silicate, namely sodium water glass with the molecular formula of Na2O·nSiO2。 Preferably, the sodium carbonate solution is dropwise added into the reaction kettle at a constant speed in the step (2), wherein the dropwise adding speed is controlled to be 0.1kg to 0.2kg of sodium carbonate per minute. The sodium carbonate is dripped at a reasonable speed to be beneficial to the formation of a uniform and stable crystal structure of the magnesium lithium silicate, the texture of the crystal structure is soft, and the obtained powder particles are fine and uniform, so that the transparency of the gelled powder in application is facilitated, and the compatibility with other components in a formula is also facilitated. More preferably, the reaction in the reaction kettle of the invention is kept boiling and continuously stirred in the whole process, and further, favorable conditions are provided for the magnesium lithium silicate to form a uniform and stable crystal structure. Optionally, after the reaction in the reaction kettle is completed, the method further comprises the following steps: cooling the reaction kettle, adding water, stirring uniformly, and filtering until the water is drained; and drying the filter cake to obtain blocky lithium magnesium silicate solid, and crushing. Wherein, specifically, the filtration is as follows: filtering water through a plate and frame filter. When the filter cake is dried, an oven with the temperature of 180-220 ℃ is preferably adopted, and the drying temperature is more preferably 200 ℃. After the massive magnesium lithium silicate solid is obtained, in some modes, the magnesium lithium silicate is crushed by a crusher, preferably into 300-350 meshes of powder, and more preferably 325 meshes of powder. Example 1 The embodiment provides a preparation method of lithium magnesium silicate, which mainly comprises the following raw materials: food grade magnesium sulfate heptahydrate (MgSO)4·7H2O), water glass (liquid, water-soluble sodium silicate, modulus ratio 3), lithium chloride (LiCl), edible sodium carbonate (Na)2CO3)； The preparation steps are as follows: a.29.6kg of magnesium sulfate heptahydrate was completely dissolved in 130kg of deionized water for use. b. In a L reaction kettle containing a condensation reflux system and a heating system, 5.18kg of lithium chloride is added, 50kg of water is added, heating and stirring are carried out, boiling is kept for 3 minutes, then magnesium sulfate solution is added, boiling is kept for 20 minutes, and the reaction kettle is kept boiling and stirring is continuously carried out in the whole production process. c. Adding 36.65kg of water glass into a dropping pot, uniformly dropping the water glass into the reaction kettle, and controlling the dropping speed so that the dropping time of the water glass is 80 minutes. d. Dissolving 7.24kg of edible sodium carbonate in 140kg of deionized water, stirring to completely dissolve, adding into a dropping tank, and controlling the dropping speed for 60 minutes. e. After the dropwise addition, the reaction kettle was kept boiling for 18 hours. f. And after the reaction is finished, cooling, adding 200kg of deionized water, uniformly stirring, filtering to remove water through a plate and frame filter, and drying a filter cake in an oven at the temperature of 200 ℃ for 4 hours to obtain a blocky magnesium lithium silicate solid. g. The lithium magnesium silicate blocks are crushed into 325-mesh powder by a crusher. The heating and boiling maintaining of the reaction kettle are realized through heat conducting oil and an intelligent temperature control system, wherein the oil temperature of the heat conducting oil is 110 ℃. Example 2 The present embodiment provides a method for preparing lithium magnesium silicate. The procedure was substantially the same as the starting material and the preparation method in example 1, except that the raw material used was water glass having a modulus ratio of 4. The other raw materials, the preparation steps and the conditions are the same. Example 3 The present embodiment provides a method for preparing lithium magnesium silicate. The process was substantially the same as the starting material and process described in example 1, except that the rate of addition of water glass in step c was different, the completion time of the addition of water glass was 100 minutes, the rate of addition of edible sodium carbonate in step d was different, and the completion time of the addition of edible sodium carbonate was 40 minutes. Comparative example 1 The comparative example provides a preparation method of lithium magnesium silicate, which adopts the following raw materials: magnesium hydroxide (Mg (OH)2) Water glass (liquid sodium silicate, modulus ratio 3), lithium carbonate (Li)2CO3) And edible sodium carbonate (Na)2CO3) (ii) a The raw materials of magnesium element and lithium element are different from those of example 1. The preparation method is basically the same as that of the embodiment 1, wherein: the amount of magnesium hydroxide was 7kg, and the amount of lithium carbonate was 4.15kg, which was the same as the amount of magnesium and lithium in example 1. Comparative example 2 This comparative example provides a method for preparing lithium magnesium silicate using the same raw materials and amounts as in example 1. The preparation method comprises the following steps: a.29.6kg magnesium sulfate heptahydrate and 5.18kg lithium chloride were dissolved in water, and then mixed with 36.65kg water glass, and the mixture was heated to boiling in a reaction vessel with continuous stirring. The reaction time was 100 minutes. b. Dissolving 7.24kg of edible sodium carbonate in 140kg of deionized water, stirring to completely dissolve, adding into a dropping tank, and controlling the dropping speed for 60 minutes. c. After the dropwise addition, the reaction kettle was kept boiling for 18 hours. e. And after the reaction is finished, cooling, adding 200kg of deionized water, uniformly stirring, filtering to remove water through a plate and frame filter, and drying a filter cake in an oven at the temperature of 200 ℃ for 4 hours to obtain a blocky magnesium lithium silicate solid. f. The lithium magnesium silicate blocks are crushed into 325-mesh powder by a crusher. The heating and boiling maintaining of the reaction kettle are realized through heat conducting oil and an intelligent temperature control system, wherein the oil temperature of the heat conducting oil is 110 ℃. Comparative example 3 This comparative example provides a method for preparing lithium magnesium silicate using the same raw materials and amounts as in example 1. The preparation method comprises the following steps: a.29.6kg of magnesium sulfate heptahydrate was completely dissolved in 130kg of deionized water for use. b. In a L reaction kettle containing a condensation reflux system and a heating system, 5.18kg of lithium chloride is added, 50kg of water is added, heating and stirring are carried out, boiling is kept for 3 minutes, then magnesium sulfate solution is added, boiling is kept for 20 minutes, and the reaction kettle is kept boiling and stirring is continuously carried out in the whole production process. c. Adding 36.65kg of water glass into a dropping pot, uniformly dropping the water glass into the reaction kettle, and controlling the dropping speed so that the dropping time of the water glass is 120 minutes. d. Dissolving 7.24kg of edible sodium carbonate in 140kg of deionized water, stirring to dissolve completely, directly adding all the materials into a reaction kettle, stirring uniformly, and reacting for 60 minutes. e. After the dropwise addition, the reaction kettle was kept boiling for 18 hours. f. And after the reaction is finished, cooling, adding 200kg of deionized water, uniformly stirring, filtering to remove water through a plate and frame filter, and drying a filter cake in an oven at the temperature of 200 ℃ for 4 hours to obtain a blocky magnesium lithium silicate solid. g. The lithium magnesium silicate blocks are crushed into 325-mesh powder by a crusher. The heating and boiling maintaining of the reaction kettle are realized through heat conducting oil and an intelligent temperature control system, wherein the oil temperature of the heat conducting oil is 110 ℃. EXAMPLES 1-2, COMPARATIVE EXAMPLES 1-3 characterization and Performance testing of lithium magnesium silicate 1. Infrared detection The prepared magnesium lithium silicate is detected by a Fourier transform infrared spectrometer (FTIR). The model is as follows: nexus, manufactured by Thermo Nicolet, USA, with a maximum resolution of 0.019cm-1The scan rate was 1 time/second. The test wavelength range is -40 cm of mid-infrared-1。 The results of the infrared test showed that the samples prepared in examples 1 and 2 of the present invention were cm-1A strong and wide absorption peak exists nearby, and the absorption peak is cm-1And in the form of-1There is also a distinct absorption peak. Wherein, cm-1Is the peak of hydroxyl hydrogen bond between hydroxyl stretching vibration and crystal layers, and is cm-1And in the form of-1The absorption peak is the shock absorption peak of absorbed water molecule-OH and the stretching vibration absorption peak of Si-O bond in the crystal lattice of magnesium silicate lithium. The above absorption peaks are all characteristic absorption peaks of magnesium lithium silicate, which indicates that the magnesium lithium silicate is really prepared by the invention. 2. Rheology test The lithium magnesium silicate powder prepared in examples 1 to 2 and comparative examples 1 to 3 was added to deionized water to prepare an aqueous dispersion of 3% by mass of lithium magnesium silicate, and the time required for gelling and the transparency thereof were recorded. And standing for 24 hours, detecting according to a conventional method, and testing the light transmittance and the viscosity of the product.

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This Open Access Article is licensed under a
Creative Commons Attribution 3.0 Unported Licence DOI: 10./C8TAB (Review Article) J. Mater. Chem. A, , 6, -

A review of magnesiothermic reduction of silica to porous silicon for lithium-ion battery applications and beyond†

Jake Entwistle , Anthony Rennie‡ and Siddharth Patwardhan *
Department of Chemical and Biological Engineering, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK. :

Yayang Product Page

Received 3rd July , Accepted 8th August

First published on 11th September

Abstract

Increasing demands for portable power applications are pushing conventional battery chemistries to their theoretical limits. Silicon has potential as an anode material to increase lithium-ion cell capacity. The associated volume change during lithiation/delithiation leads to a decline in capacity during cycling and low lithium diffusion rates within silicon limit high rate performance. Porous silicon can potentially address the poor cyclability and rate capabilities simultaneously by minimising stresses and providing smaller silicon substructures for lithium diffusion. Template assisted synthesis and magnesiothermic reduction of silica to silicon offers a facile and scalable route for the production of porous silicon structures even when using a non-porous feedstock. This review collates the available literature concerning the effects of reaction conditions through the reduction reaction. We highlight that it is important to report in detail all reaction conditions and complete characterisation of both the reactant and the product. The battery performance of these porous silicon structures is discussed and future research directions are identified. These outcomes will enable the identification of a clear design pathway for the bespoke production of porous silicon.

1 Silicon anodes

Rechargeable batteries play an important role in portable electronics and are increasingly being incorporated on a larger scale into electric transportation. Lithium-ion batteries (LIB) have become the chemistry of choice due to their combination of good energy and power density.1,2 The development of lithium-based rechargeable batteries with high energy and power density, improved safety and low cost is highly desirable.3 Silicon offers a significant volumetric and gravimetric energy density advantage over conventional LIB anode materials such as graphite. At room temperature the theoretical specific capacity of Si is mA h g−1 (and mA h ml−1)4 corresponding to the formation of the Li15Si4 phase. This is substantially larger than that of graphite 371 mA h g−1 (830 mA h ml−1). Silicon's low operational voltage, high earth abundance and low toxicity are additional benefits.

In conventional electrode materials, lithiation and delithiation follow an intercalation mechanism where lithium ions are inserted and extracted from interstitial sites of the host material. This results in relatively small structural changes and good capacity retention. Silicon, in contrast forms an alloy with lithium, which involves the breaking and reforming of chemical bonds with the host structure upon every cycle. The large number of lithium ions inserted into silicon causes large volume changes up to 280%.5 Such a volume change within composite electrodes leads to structural damage, isolation of active material and ultimately loss of capacity. This poses a significant challenge for the development of long cycle life high capacity silicon anodes.

1.1 Understanding silicon lithiation

At room temperature, lithiation does not occur spontaneously according to the thermodynamic phase diagram.6 Silicon can either be crystalline or amorphous prior to lithiation. If silicon is crystalline then the first lithiation is via a two-phase mechanism between Si and amorphous LixSi phases which are separated by a short reaction front of a few nm.7 There is a large activation energy required to break Si–Si bonds within the crystalline silicon matrix and therefore a high concentration of lithium ions is required to weaken the Si–Si bonds at the reaction front, leading to favourable lithiation kinetics and two-phased behaviour.7,8 This amorphous LixSi phase is found to crystallise to Li15Si4 around 100 mV vs. Li/Li+ (ref. 7); however, this may be higher depending on factors such as the crystallite size.9 This results in a galvanostatic voltage profile with a voltage plateau around 0.1 V vs. Li/Li+, suggesting that a two-phase mechanism dominates the lithiation process.

During the first delithiation another two phase mechanism occurs in reverse, eventually yielding amorphous silicon. After the first cycle, lithiation subsequently occurs into amorphous Si. The conversion of crystalline silicon through lithiation over the initial cycles was studied through in situ XRD6 and a physical model of amorphous silicon lithiation has been developed based on 7Li NMR investigations.8In situ TEM observations for lithiation of both crystalline and amorphous silicon indicate a critical size for nanoparticles, beyond which they crack, 150 nm diameter for crystalline silicon and 870 nm for the amorphous phase.10,11 Volume changes of up to 300% in magnitude have been studied in silicon thin films with AFM and were shown to be roughly linear with Li content,12 with the linear increase with the lithiation level also being modelled.4

Within the constrained environment of a composite electrode, the volume expansion of silicon becomes an issue for three main reasons as depicted in Fig. 1.13 Firstly considering Fig. 1(a), stresses within individual particles lead to fracturing and eventually pulverisation. Fig. 1(b) shows that the impingement of expanding material in the electrode can lead to fragmentation on a larger scale, disconnecting sections of the electrode. In each case, active material becomes electrically isolated and no longer contributes to the electrode capacity. Fig. 1(c) illustrates the case when constant expansion and contraction leads to the cracking of the Solid Electrolyte Interphase (SEI) layer, exposing fresh active material and causing further breakdown of the electrolyte. The thickening of the SEI can increase the internal resistance of the cell and consumes lithium from the electrolyte.

Another challenge is the relatively low lithium diffusion rates within silicon, 10−10 to 10−11 cm2 s−1,14,15 compared with the range 10−6 to 10−11 cm2 s−1 reported for graphite electrodes.16–18

During lithiation, the process is complete when the lithium-rich Li15Si4 phase is formed at the surface of the silicon electrode; similarly delithiation finishes when the lithium ions are extracted from the very outer surface layer. This arises as the potential of the cell is determined by the chemical potential difference of lithium ions between the two electrodes. If the potential drops below the operational voltage window, then regardless of whether inner silicon has participated in lithiation/delithiation, the process will be stopped. This low diffusion rate poses a kinetic barrier to achieving the theoretical capacity of silicon and is pronounced with larger silicon grain sizes. At higher current rates, the voltage drop across the internal resistance of the battery can compound, leading to the theoretical capacity not being reached.19

1.2 The role of porous silicon

Porous morphologies can potentially address the challenges of volumetric expansion and slow lithium diffusion. Porous silicon can expand into its own pore volume, thus limiting stresses on the material. Stress generation upon lithiation has been modelled for lithium insertion materials20,21 which in turn have been applied to models of lithiation in porous silicon structures.19,22,23 A comparison of hollow and solid amorphous silicon nanospheres of the same silicon volume modelled by Yao et al. showed that the maximum tensile stresses experienced by a hollow sphere is significantly lower vs. lithiation time, 83.5 vs. 449.7 MPa respectively (Fig. 2(A)).22 Ge et al. simulated the effect of pore size and porosity in the range of initial pore sizes 1–9 nm, showing that lower porosities lead to higher induced stress and smaller pore sizes result in higher maximum hoop stresses around the pore (Fig. 2(B)).19 Li et al. also showed a linear decrease in the porous particle volume change by increasing the void volume fraction for materials containing 10 and 25 nm radius ordered pores. Fig. 2(C) shows that increasing the void fraction to 60% should keep the overall particle expansion within the narrower volumetric expansion range of 75 to 150%.23

Porous silicon with a high surface area can increase the accessibility of the electrolyte to silicon surfaces, shortening lithium diffusion lengths and increasing available capacity at higher rates. Porous structures with thin walls and silicon substructures can shorten the diffusion length of lithium within silicon. Polycrystalline porous structures with small silicon domains are hypothesised to have a higher resistance to fracture during lithiation similar to the observed strong size dependence of fracture of silicon nanoparticles.10 An in situ TEM observation has shown that the critical particle diameter for fracture in porous particles reaches nm.9 Additionally, in situ TEM and dynamic simulation of the lithiation behaviour of porous silicon particles found that the smaller domains of porous particles disfavour the crystalline c-Li15Si4 phase upon full lithiation, providing a more favourable stress evolution on expansion (Fig. 3).9 Additionally, it was shown that these porous particles lithiated in an end-to-end fashion (Fig. 3(e–h)), as opposed to larger nanoparticles which lithiated in a surface-to-centre manner (Fig. 3(a–d)).9

High surface area porous silicon structures will however generate a larger SEI simply because of the increased electrode area in contact with the electrolyte. This could be a significant drawback when coupled with the volume expansion of silicon and relative instability of the SEI. Surface coatings and particle encapsulation mitigate these issues and are described in Section 3.

1.3 Synthesis/fabrication of silicon

The worldwide annual production of silicon metal amounted to tons in .28 The industrial scale synthesis of metallurgical grade silicon is typically achieved by the reduction of silica with carbon in electric arc furnaces at temperatures over °C.29,30 A common industrial method of refining metallurgical grade silicon is chemical vapour decomposition (CVD), see Fig. 4(b). CVD methods usually require reaction temperatures of °C in the presence of hydrogen gas. Producing the volatile silicon precursors needed also requires a reaction with hydrochloric acid at 350 °C.29 The other major refinement methods rely on the crystallisation of molten silicon, again requiring high temperatures > °C to achieve molten silicon, Fig. 4(a). The development of environmentally friendly, low cost and scalable production processes for high performance silicon anodes is essential.

There has been great progress in developing silicon anodes with a wide range of nano-structuring techniques being employed and extensively reviewed elsewhere.13,24–26 High capacities and good cyclability have been demonstrated using nanoscale silicon anode structuring.13,24,26 Porous silicon structures have also been reviewed by Ge et al.25 However, nanoscale engineering often requires a high degree of precision synthesis and can involve aggressive reaction conditions. Using energy intensive synthesis techniques will likely lead to high cost of materials and the inability to produce materials on an industrial scale.27

Magnesiothermic reduction can offer a facile method for the bulk synthesis of porous silicon with varying degrees of structural control. Fig. 4 highlights the benefits of this single step reduction method from silica to silicon in comparison to existing technologies. This review collates the available literature concerning the influence of reaction conditions on the magnesiothermic reduction reaction as well as highlighting gaps in the current understanding. The use of porous silicon produced through magnesiothermic reduction as an active material for anodes in LIBs is the focus of this review, but potential applications may be much further ranging with impact in fields such as photoluminescence,31 solar power,32 photocatalysis,33 drug delivery34 and catalysis support.35

2 Magnesiothermic reduction

2.1 Background

Magnesiothermic reduction of silica (SiO2) has the potential to produce porous silicon materials at lower temperatures than conventional silica reduction methods.36 As the melting point of silicon is °C, carbothermal reduction at °C is not suitable for maintaining the silica template structure to give silicon pseudo-morph analogues. Magnesiothermic reduction has been demonstrated to produce silicon structures from silica in the temperature range of 500–950 °C, permitting template assisted design of silicon structures. This method has shown the ability to preserve intricate features in the silicon produced as small as 15 nm.37 The high diversity and robust understanding of silica chemistry allow for the possibility of creating a wide range of silica template structures with tailored geometries. This reduction entails the reaction of magnesium with silica resulting in an interwoven composite product of magnesia (MgO) and silicon (reaction (1)).SiO2 + 2Mg → Si + 2MgO(1)

Magnesia is easily removed with HCl, leaving a silicon replica behind that possesses a higher surface area than the starting template. Fig. 5 summarises the two step reduction–etching process. The interwoven nature of this morphology is crucial to allow the removal of magnesia in this way. The formation of the interwoven aggregate morphology of the product Si and MgO is thought to relate to the stability of the reaction interface and flux of reactants across the product phases.38

The Gibbs energy of magnesiothermic reduction is negative for the entire temperature range 0– °C and this indicates that the reaction is exergonic.40,41 The enthalpy of reaction (1) is exothermic and has significant ramifications as discussed below.41 The melting point of magnesium is 650 °C, and the vapour pressure of Mg at 428 °C is 1 Pa. This enables solid magnesium and silica to be placed separately in the reaction vessel and the magnesium gas to diffuse to reduce the silica and/or liquid magnesium to flow over the silica.36,42

It is important to note the side reaction can reduce the yield of silicon through the formation of magnesium silicide (reaction (2)). A number of studies when placing the magnesium and silica separately and relying on the gas–solid type reaction report the silica close to the source being reduced to Mg2Si and a middle region forming Si, with further displaced sample showing no reduction and remaining as SiO2.36,40,42,43 Mg2Si formation reduces the yield of the reaction and affects the product morphology upon removal which occurs simultaneously with MgO removal in the HCl wash. Mg2Si is not thermodynamically stable in the presence of SiO2 and therefore its formation is due to kinetic limitations.44 Silanes are produced by the reaction of magnesium silicide with acid. They react spontaneously with air.

Si(s) + 2Mg(g) → Mg2Si(s)(2)

Fig. 6 displays key reaction variables with respect to the stages of the reaction. Below we aim to summarise and evaluate critically the literature findings (ESI Table S1† displays in detail the key parameters identified in the literature).

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A number of challenges remain in the development of a controllable magnesiothermic reduction method for porous silicon production, primarily the preservation of the silica template morphology in the silicon analogue, which underpins the rationale of the method. Another challenge is control over the extent of the reaction to fully reduce the silica avoiding the need for the undesirable process of HF etching, while simultaneously maintaining the silicon yield by avoiding overreduction to form Mg2Si.

In order to design the porous silicon product that is desirable for anode applications, the feedstock and the process parameters governing the magnesiothermic reduction process and their effects on the properties and performance of silicon should be understood. In Section 2.2 we discuss these key features and their effects on the product silicon.

As evident from the foregoing, a truly comparative literature review is difficult as most studies reduce just one specific silica material under set reaction conditions and report a limited number of variables. Magnesiothermic reduction reaction conditions are not all comparable from one study to another. As a result, the silicon product morphology can vary for the same silica templates between studies.

2.2 Reduction conditions

The key control parameters for the reduction reaction are: reaction time, temperature and temperature ramp rate, reactant proximity and morphology, and molar ratio of reactants. Conventionally, magnesiothermic reduction is carried out under a flowing 95% argon or a nitrogen atmosphere, although some studies have performed the reduction under vacuum to promote a higher vapour pressure of magnesium.45–47 Another source of magnesium gas can also be achieved from the volatilization of Mg2Si.48,49 All these parameters have significant effects on the silicon product. However, a comprehensive study on the effect of reaction conditions on the product properties is lacking in the literature. In Section 2.2 below we aim to highlight key studies where quantitative conclusions can be drawn between reaction conditions and product silicon properties.

2.3 Precursor silica

A wide range of biological and synthetic silica sources are available and have been reduced via magnesiothermic reduction for LIBs. Both sources, as shown in ESI Table S1,† have the ability to produce porous silicon with a variety of pore properties. It should be noted that biologically derived silica such as rice husk and bamboo silk requires the removal of organic components and commonly needs acid leaching of metal impurities. Synthetic silica sources can be equally time consuming as well as resource and energy intensive to produce (see Fig. 4(c)). In the striving to produce battery materials in a more economical and environmentally friendly way, the choice of silica template is also significant.

Table S1† summaries the key parameters of silica before and silicon after reduction where these data were available. In general, many of the reduction reactions produce mesoporous silicon with a range of surface areas between 24–350 m2 g−1 and pore volumes 0.11–1.1 cm3 g−1. Interestingly these mesoporous properties also appear in samples which initially were non-porous such as sand59 and silica spheres.51,52 In these cases the nature of the silicon-magnesia interwoven aggregate introduces porosity to the structure through the removal of the magnesia phase. The same effect is seen on non-porous starting silica/silicon used in the ‘deep reduction partial oxidation’ method.63,64 It appears that when the overall template morphology is maintained, and if reaction temperatures can be controlled sufficiently below the silicon melting point, pores in the mesopore region and overall pore volumes will increase irrespective of the porosity of the precursors. This is somewhat expected as the magnesia phase occupies ∼65% of the volume in the product structure and oxygen is being removed from the initial template.48

Rice husks have been reduced by magnesiothermic reduction in a number of studies.42,50,61,62,68 Table S1† shows how the initial rice husk precursors have similar SSA and pore volumes, but the purity of the silica varies. Where reported, upon reduction, the pore volumes increase in all studies with surface areas remaining similar to or lower than that of the starting silica, with the exception of ref. 61 where the surface area and pore volume are dramatically decreased from 234 m2 g−1 to 42 m2 g−1 and 0.43 cm3 g−1 to 0.31 cm3 g−1. In that study, a lower reaction temperature of 500 °C may contribute to the pore properties not following the trend. Note that this is below the onset temperature reported at 540 °C by Larbi et al.32 who also reduced silica rice husk. This example highlights the lack of understanding in the evolution of magnesiothermic reduction reactions. Additionally, this shows how specific factors in each case can also contribute to significant variation in product properties; these factors include furnace design, crucible design and packing and mixing of reactants.

Table S1† shows a number of examples where the magnesiothermic reduction conditions have had a much greater effect on the silicon morphology than the template used.46,69,70 Kim et al.71 utilised a change of morphology to obtain their desired structure; they reduced vertically aligned mesoporous silica to obtain 10 nm silicon nanoparticles dispersed on graphene sheets. Similarly, Zhu and Wu utilised magnesiothermic reduction to produce silicon nanoparticles on graphene sheets.72,73 Although not initially the function of this method, it has proven to produce novel materials with good properties for lithium-ion battery applications and potentially beyond.

2.4 Summary

In this review it has been clearly shown how individual reaction parameters can affect the silicon product properties. In reality the interplay of reaction parameters with each other may add more complexity to the relationship between reaction conditions and product properties. However, the scientific understanding of the key parameters collated above should be able to provide valuable insight into the importance of reaction conditions on product properties. Table 1 below summarises the key effects of the ramp rate, temperature, time and molar ratio.

A wide variety of silica precursors have been studied through magnesiothermic reduction. By reviewing the relevant literature, it is clear that under circumstances where the heat accumulation has been mitigated, the reduction of silica will introduce mesopores into the silicon product and increase the overall pore volume. This is observed for precursors which initially have micropores and mesopores and even for non-porous precursors. A strong indication of heat accumulation, causing the reaction to approach or exceed the melting point of silicon, is a typical macroporous product with spherical pores around 200 nm.46,66 These two effects can be described by the nature of the interwoven aggregate silicon/magnesia product phase. At lower temperatures, smaller magnesia phases (in the mesopore size range 2–50 nm) are formed interwoven with silicon, with elevated temperatures causing the aggregation of magnesia crystallites into larger grains (in the macropore range ∼ 200 nm).

3 Anode performances

The studies using rice husks as a silica source presented in Table 2 are examples of how varying magnesiothermic reduction conditions (ESI Table S1†) can greatly affect the SSA and pore volumes of the silicon product. In general, lower surface areas and pore volumes result in lower capacities and poorer capacity retention.42,61,62 The example of rice husk reduction from Liu et al.42 shows the benefits of porous silicon, with a high stable capacity of mA h g−1 and reasonable capacity retention over 300 cycles. Factors such as the electrode composite and electrolyte additives also likely played a key role in performance parameters. Interestingly the best performing rice husk derived silicon electrode was formed using a polyvinylidene fluoride (PVDF) binder; which has previously been shown to be inferior to silicon.74,75 The high porosity of these samples is likely to limit the overall particle expansion, as discussed in Section 1.2, negating the need for more flexible binders.

SBA-15 is a silica with mesopores, a biphasic system of ordered hexagonal arrays of pores.76 A number of studies have used a magnesiothermic reduction method with SBA-15 to achieve ordered mesoporous silicon for anodes.39,54,63 The SBA-15 used is either produced in-house39 or purchased from different suppliers34,54,63 introducing some discrepancies between studies. The best performing SBA-15 silicon analogue is reported by Jia et al.,39 especially when considering fast charging rates as shown in Fig. 8(c). The lower surface area and larger pore diameters could perhaps be responsible for this success, as they favour less SEI formation and larger pores experience smaller stresses on expansion. This material performs well at mA h g−1 for 100 cycles with 94.4% capacity retention when using a carbon coating (Fig. 8(b)). In this case the porous silicon is coated with a 4 nm layer of amorphous carbon via chemical vapour deposition filling some pores and decreasing the surface area. With the carbon coating negating the negative effects of the increased silicon surface area causing excessive SEI formation, it can be seen that this mesoporous material has very attractive properties for LIB applications. A number of studies demonstrate the attractive properties of combining a carbon coating and porous silicon in this manner.51,77–80

The study of Liang et al.63 using ‘deep reduction and partial oxidation’ is an example for the comparison of the effects of various porous silicon structures on anode performance. The properties and performance of the four materials are summarised in Table 2. The two best performing materials are the reduced aerogel and sand. These two materials have the highest and lowest surface area among the four materials and the smallest and largest pore diameters respectively. This stands as an important example showing that SSA and pore properties are not the only key parameters determining porous silicon's success as an anode material. Liang et al.63 reported that the morphology of the SBA-15 and diatom, which is an aggregation of isolated particles, adversely influences the cycle life. The route created a 3 nm passivation layer of silica over the surface of the porous silicon. The aggregation of particles is therefore not beneficial as ionic and electronic conduction is hindered through silica layers. Post cycling TEM results of the diatom, SBA-15 and sand silicon appear somewhat different to those reported by Shen et al.9 and Liu et al.,42 further indicating that upon cycling these samples, with passivated silica layers, perform differently.

4 Challenges and opportunities

(1) It is clear that the surface area and porosity do not completely govern the success of silicon as a LIB anode. For this reason, it is imperative that researchers publish detailed characterisation of material properties such as surface areas, pore volumes, pore size distributions, crystalline properties and particle morphology.

(2) The evolution of the pore structure in porous silicon during lithiation has not been studied in great detail. With the ability to see which pore structures are beneficial to improved performance, an in situ or ex situ method of evaluating the pore structure vs. cycle life would significantly improve the understanding in this area. Such insight would give researchers the information necessary to use the versatile magnesiothermic reduction to provide these porous material properties on a bulk scale.

(3) Cycling parameters and other testing variables such as electrolyte additives81 and binders75,82 are well known to improve the capacity and cycle life of silicon anodes. Testing criteria for the fabrication of electrodes and cells should be reported in detail. Equally as these variables have shown significant improvements in non-porous materials their effect on porous materials is still to be quantified.

Like for like comparison between reports may never be possible due to experimental inconsistencies. However, for better comparison in future, researchers should strive to publish porous silicon performance vs. a silicon standard material.

5 Conclusion

Silicon has outstanding features as an anode material for LIBs, primarily its high specific and volumetric energy density. The major drawback is hinged upon the large volume change associated with lithiation causing the reversible cycling capacity to be poor. The properties of porous silicon have been shown to be advantageous in increasing the capacity and extending the cycling life. If performed effectively magnesiothermic reduction offers a relatively facile bulk synthesis route to porous silicon materials with morphological control over the silicon product.

Synthesis via magnesiothermic reduction has been shown to offer the possibility of using templated silicon analogues from the vast catalogue of synthetic and natural sources. This method provides an instrument to study the varying effect of the porous silicon morphology on the anode performance. The reduction is a highly exothermic reaction and efforts must be made to avoid excessive rises in the reaction temperature. A number of studies have shown that slower ramping rates and thermal moderators can overcome excessive heat accumulation. Additionally, two step synthesis with lower enthalpies has been shown not only to avoid template destruction but provide oxide layers linked to improved cycling performance. The exothermic nature of the reaction means that the true reaction temperature can be well above the set conditions, and reported in excess of °C. It can therefore be difficult to strictly control and study the effect of temperature on the reaction. The reports of initiating the reaction at higher temperatures and then lowering the reaction temperature are interesting regarding both control and efficiency aspects.

The magnesiothermic reduction template assisted method leads to a mainly mesoporous silicon product with a similar shape to the initial template, although mesopores can be introduced into samples even if the templating structure is non-porous. To better understand the processes occurring under magnesiothermic reduction conditions studies should report in detail all the reaction criteria recommended above. Additionally, complete characterisation of both the reactant and product structures is needed to see how reaction conditions may affect different templates.

Key to assessing the effectiveness of porous silicon as an anode is understanding the evolution of the porous structure during lithiation. Methods such as TEM have started to reveal these behaviours.9,42,63,83 More analysis of porous silicon would be a valuable addition to understand how the pore structure evolves under cycling.

The high surface areas and pore volumes along with the nanocrystalline properties of magnesiothermically reduced silicon are advantageous for a number of additional applications in fields such as photoluminescence,31 solar power,32 photocatalysis,33 drug delivery,34 and catalysis support.35 Understanding the magnesiothermic reduction mechanism of producing porous silicon is crucial for the expansion of this technique. A clear design pathway should be identifiable from feedstock silica to desired silicon properties.

Conflicts of interest

There are no conflicts to declare.

Abbreviations

LIBLithium-ion battery CVDChemical vapour deposition

Acknowledgements

This work is supported by the EPSRC EP/L/1 and EP/R/1 grants.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10./c8tab‡ Present address: Faradion Limited, The Innovation Centre, 217 Portobello, Sheffield, S1 4DP.This journal is © The Royal Society of Chemistry