Recent developments in synthesis of xLi2MnO3 · (1 − x)LiMO2 (M = Ni, Co, Mn) cathode powders for high-energy lithium rechargeable batteries

Introduction

From its first commercial introduction by Sony in 1991, the LiCoO₂ cathode has been widely used in portable electronics due to its excellent rate capability, cyclability, and high-tap density (Ying et al., 2004; He et al., 2006). Although the theoretical specific capacity of LiCoO₂ electrodes is 273 mAh g⁻¹, the practical specific capacity is ~140 mAh g⁻¹ due to high-surface reactivity and instability of the delithiated Li_1−xCoO₂ (x > 0.5) (Thackeray et al., 2007; Nagai et al., 2011). Cobalt, however, is relatively expensive and somewhat toxic, thus, there is a great need to find new cathode materials with superior capacity, energy, safety, and lower cost.

Argonne National Laboratory initiated research to develop a family of Li₂MnO₃-stabilized LiMO₂ (M = Mn, Ni, Co) cathodes for lithium-ion batteries (Kim et al., 2002, 2004; Thackeray et al., 2005). These electrode materials have similar layered structure to LiCoO₂, but possess superior stability, which allows the extraction of higher quantities of lithium ions during charge over a wider operating potential range, such as 2.0–4.6 V. As a result, practical specific discharge capacity of more than 200 mAh g⁻¹ can be achieved at an average discharge potential of ~3.6 V vs. Li⁺/Li°. In addition, thermal stability of these electrodes is significantly greater than that of LiCoO₂. Furthermore, less cobalt usage leads to lower material cost and environmental hazard.

There are excellent reviews (Thackeray et al., 2005, 2007; He et al., 2012; Yu and Zhou, 2013) describing molecular structure, thermal stability, electrochemistry, cycling stability, and other physical as well as electrochemical properties of xLi₂MnO₃·(1 − x)LiMO₂ for cathode application. However, there is no review, which explains the synthetic procedures of these cathode materials. Because the synthesis method greatly determines the material properties and its commercial feasibility, a systematic review of synthesis procedures of xLi₂MnO₃·(1 − x)LiMO₂ powders is crucial, especially for researchers who are entering high-energy lithium-ion battery research. In this review, we discuss many synthesis pathways of xLi₂MnO₃·(1 − x)LiMO₂ powders for cathode application in high-energy lithium-ion batteries.

Solid-State Synthesis

Solid-state synthesis is a conventional method for the preparation of electrochemically active materials for lithium-ion batteries. It is considered as a simple and scalable method. However, several choices of precursors, temperature profiles, cooling modes … should be identified and optimized. The solid-state process includes several successive steps such as mixing, milling, grinding, pelletizing, annealing, and quenching. In many cases, ball milling devices may be used for the purpose of fast and effective mixing and grinding. However, simple grinding and mixing by mortar and pestle are effective and widely accepted. Since reactions are controlled by solid-state diffusion, time, and energy consumption are important factors. For preparation of xLi₂MnO₃·(1 − x)LiMO₂ (M = Ni, Co, Mn), the precursors consist of transition metal salts (acetates of Ni²⁺, Co²⁺, Mn²⁺) (Wang et al., 2012; Yu and Zhou, 2012; Yu et al., 2012a,b, 2013a), manganese–nickel–cobalt hydroxides (Kim et al., 2002, 2004; Johnson et al., 2007; Li et al., 2011; Xu et al., 2011; Zhang et al., 2012), or carbonates (Deng et al., 2010; Croy et al., 2011; Koenig et al., 2011; Wang et al., 2011), and a lithium-containing compound (usually lithium hydroxide). The stoichiometric mixture is initially decomposed at 480–600°C for 3–15 h, then pelletized and calcined at 850–1000°C for 10–24 h in air. The sample can be rapidly quenched (in air or liquid nitrogen) (Kim et al., 2002; Ito et al., 2008; Wu and Manthiram, 2009; Madhu et al., 2010; Li et al., 2011; Zhang et al., 2012) or furnace cooled (Wang et al., 2012).

Solid-State Reaction in Combination with Co-Precipitation

Most xLi₂MnO₃·(1 − x)LiMO₂ samples are prepared by a combination of co-precipitation and solid-state reactions (or mixed hydroxide method), following the pioneer work at Argonne National Laboratory (Kim et al., 2002, 2004). Scanning electron microscopy images of a typical product is shown in Figure 1 (Li et al., 2011), which represent the non-uniformity of the solid-state synthetic products. The manganese–nickel–cobalt hydroxide precursor is prepared by co-precipitation of transition metal aqueous solutions (nitrate or acetate of Ni²⁺, Co²⁺, Mn²⁺) in a basic aqueous solution (sodium, potassium, or lithium hydroxide) (Kim et al., 2002; Kang et al., 2006; Johnson et al., 2007, 2008; Park et al., 2007a; Wu and Manthiram, 2009; Li et al., 2011; Xu et al., 2011). In order to prepare the hydroxide precursor with uniform particle size, polyvinylpyrrolidone (PVP) and ethylene glycol (EG) are used as dispersants (Xiang et al., 2013). PVP hinders the growth space of particles due to micelle formation. EG limits the growth rate of particles because of the high viscosity. Subsequently, the dried material is mixed with lithium hydroxide to produce the starting mixture for solid-state reactions.

FIGURE 1

Figure 1. SEM pictures of Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ materials at different magnifications. (A) ×5k; (B,C) ×50k. Reproduced with permission from Li et al. (2011).

Deng et al. (2010) reported the preparation of Li_(1+x)Ni_0.25 Mn_0.75O_(2.25+x/2) (where x = 0.32–0.65), which contains the Li₂MnO₃ phase, by using all carbonate precursors. A spherical Ni_0.25Mn_0.75CO₃ precursor is first prepared by co-precipitation, followed by solid-state reaction between that precursor and a stoichiometric amount of Li₂CO₃. Similarly, 0.5Li₂MnO₃ 0.5LiCoO₂ is synthesized by solid-state reaction between (Co_0.5Mn_0.5)CO₃ and Li₂CO₃ (Croy et al., 2011). (Co_0.5Mn_0.5)CO₃ is prepared beforehand by mixing a solution of cobalt and manganese sulfate with ammonium bicarbonate solution. In addition, Ni_xMn_1−xCO₃ can be prepared from nickel and manganese sulfate, sodium carbonate, and ammonium hydroxide (Lee et al., 2006; Ito et al., 2008; Koenig et al., 2011; Wang et al., 2011). Another transition metal precursor is NiMnO₃, which can be prepared by heating nickel manganese double hydroxides at 600°C for 4 h, then NiMnO₃ is mixed with lithium hydroxide for the subsequent solid-state reactions (Ohzuku et al., 2011). Using a combination of co-precipitation and solid-state reaction techniques, higher homogeneity is achieved, thus, improved discharge capacity such as 270 mAh g⁻¹ at 0.1 C in the voltage range of 2–4.6 V was reported (Lee et al., 2006). However, adding several steps to the conventional solid-state reaction leads to time and cost increases that may hinder scale-up and commercialization.

Solid-State Reaction

Pure solid-state preparation of xLi₂MnO₃·(1 − x)LiMO₂ has been practised by Zhou’s group (AIST, Japan). By simple solid-state reactions of lithium hydroxide and transition metal acetates, cathode materials with sufficient high-specific capacity (~225 mAh g⁻¹ at 0.1 C, 2–4.8 V) can be achieved (Yu et al., 2012b). (1 − x − y)LiNiO₂·xLi₂MnO₃·yLiCoO₂ samples can be synthesized by solid-state reactions of all acetate precursors (Madhu et al., 2010). The optimized sample exhibits a discharge capacity of 244 mAh g⁻¹ at C/15 rate in the potential range of 2–4.6 V. Oxalic acid can be used as a precipitant in the precursor mixture in order to produce dry precursor and to obtain higher homogeneous distributions of transition metal ions (Wang et al., 2012).

Sol-Gel Method

The sol-gel method appears to be the another good choice for the preparation of xLi₂MnO₃·(1 − x)LiMO₂ cathode materials. This method produces high purity and homogeneous products but is time consuming due to aging and drying. Stoichiometric amounts of lithium acetate, nickel acetate, cobalt acetate, and manganese acetate (or nitrate salts) are dissolved in distilled water (Wu et al., 2010; Jarvis et al., 2012; Jiang et al., 2013). An additional solution containing chelating agent such as citric acid (Wu et al., 2010), glycolic/tartaric acid (Kang et al., 2007), or citric acid/ethylenediaminetetraacetic acid (EDTA) (Jarvis et al., 2012) is added to the acetate solution under vigorous stirring to form a highly viscous gel. The solution pH is adjusted to ~7.5 using ammonium hydroxide. Water is evaporated at 70–80°C. The gel is decomposed at 450–500°C for 5–12 h and the solid product is ground after cooling. The resulting powders are pressed into pellets, heat treated at 800–900°C for 10–12 h. The final powder consists of nanosized primary particles, which are ~200–500 nm.

Combustion Method

Simple combustion is a popular method to prepare the lithium-rich layered cathode materials (Park et al., 2003, 2007b; Wu et al., 2004; Hong et al., 2005; Fu et al., 2014; Shen et al., 2014). In comparison with mixed hydroxide (solid-state reaction in combination with co-precipitation) and sol-gel method, combustion method is simpler and less expensive (Park et al., 2003). Precursors must contain at least one acetate salt, other precursors can be nitrate (Park et al., 2003, 2007b; Hong et al., 2005) or hydroxide (Wu et al., 2004). Stoichiometric amounts of Li, Ni, Mn, and Co sources are dissolved in distilled water under continuous stirring and evaporating at 80–100°C on a hot plate. The molar ratio of acetate to nitrate can be adjusted to 3:1 to keep the combustion condition stable (Hong et al., 2005). A viscous gel is obtained after evaporation of excess water. In case all precursors are acetates (Fu et al., 2014), mannitol can be added and pH is adjusted to 2 using concentrated nitric acid to assist the gel formation. The resulting gel is fired at 400°C for 30 min to 1 h in air. The obtaining ash-like powder is ground and heated at 500–700°C for 3 h in air, then reground and heated again at 900°C in air for 5–12 h. Finally, it is quenched to room temperature.

Other Synthesis Routes

Co-Precipitation

Similar to the sol-gel method, co-precipitation offers uniform particle size distribution as well as high-phase purity. However, few papers (Gim et al., 2012; Min et al., 2013; Yu et al., 2013b) described this method, which may be due to the complicated procedure and the difficulty in controlling the chemical composition. In a typical synthesis, transition metal acetate precursors and lithium hydroxide are dissolved separately in distilled water. The hydroxide co-precipitation of the transition metals is facilitated by slowly combining the aqueous lithium hydroxide solution with the solution containing transition metal precursors at room temperature over an extended period of time (e.g., 24 h) (Gim et al., 2012). The resulting precipitate is dried between 85 and 120°C, followed by a grinding period. The powders are then heated at 600°C for 3 h to decompose the organic compounds, then calcined at 900°C for 12 h to promote crystallization, and finally quenched (Min et al., 2013).

If lithium carbonate is used as the lithium source instead of lithium hydroxide (Yu et al., 2013a,b), the precursors are mixed in a basic solution containing ammonium heptamolybdate tetrahydrate. The precipitate is dried at 100°C and annealed at 950°C for 5 h in air.

Spray Pyrolysis

Spray pyrolysis (SP) is well known as a continuous and single-step preparation method for the synthesis of fine homogeneous and multicomponent powders. In comparison with the solid-state and sol-gel methods, SP is simpler and faster. In addition, the particle size distribution is typically narrow and controllable from the micrometer to submicrometer order, the purities of the products are high and the compositions of powders are easily controllable. Recently, the first paper reporting the preparation of xLi₂MnO₃·(1 − x)LiMO₂ by SP (Zhang and Axelbaum, 2012) was published. It is expected that more reports employing this method of synthesis will be available since it requires time to master this relatively new and special technique.

In a typical synthesis, the precursor solution is prepared by dissolving stoichiometric amounts of lithium nitrate and transition metal nitrates in deionized water. An air-assisted nebulizer aerosolizes the precursor solution to produce fine micrometer droplets. The droplets are transferred into a tubular furnace by a constant flow of air. Temperature of the furnace is held at 700°C to ensure full decomposition of the nitrate precursors. The powders are collected by a filter, and then annealed at 700–800°C for 2 h.

Electrospinning

This method was adapted by Hosono et al. (2013) to prepare hollow nanowires of 0.5Li₂MnO₃·0.5LiNi_1/3Co_1/3Mn_1/3O₂. An aqueous solution containing lithium acetate, transition metal acetate precursors, and polyvinyl alcohol are dissolved and then aged at 90°C for 1 h in an aqueous solution containing methanol and acid acetic. The precursor solution is loaded on a syringe, which connects to a metal needle. Under a voltage of 20 kV across the aluminum foil collector and the metal needle, polymer wires containing the metal salts can be obtained. Then, the desired material with wire structure could be achieved by heat treatment at 800°C in air. This method may be good for laboratory experiments to produce one-dimensional materials, but may require sophisticated high-voltage instruments for scale-up purposes.

Reactive Mechanochemical Synthesis

The previously described solid-state method often delivers products with large particle sizes (up to micrometer) with broad distribution, uncontrollable particle growth, and agglomeration. To overcome this problem, reactive mechanochemical synthesis is included in the synthetic procedure. This method is effective to prepare composite with a more uniform distribution of precursor components. There are several parameters (e.g., mass of balls/sample, rotation speed, pressure, temperature, the presence of wetting agents …) that can be controlled to deliver effective synthesis.

Mechanochemical process was utilized by Kim et al. (2012) to prepare the xLi₂MnO₃·(1 − x)LiMO₂ (M = Mn, Ni, Co). Li₂MnO₃ and LiMO₂ (M = Ni, Mn, Co) can be incorporated together with controlled molar ratio thanks to the mechanochemical process (Figure 2). This incorporation of the two phases is apparently important because LiMO₂ can be stabilized and Li₂MnO₃ is the source of excess lithium.

FIGURE 2

Figure 2. HRTEM images: (A) 0.5Li₂MnO₃·0.5LiMO₂ (B) heat treated 0.5Li₂MnO₃·0.5LiMO₂. Reproduced with permission of Kim et al. (2012).

In a typical synthesis, precursors (Li₂MnO₃ and LiNi_0.5Co_0.2 Mn_0.3O₂) and acetone are well mixed by using a planetary mill running at 350 rpm for 3 h. The composites are then heated at 1000°C for 10 h. Heat treated secondary particles are spherical. The performance of batteries prepared from this cathode material (discharge capacity is ~200 mAh g⁻¹) is lower than those prepared by the solid-state reaction. The reasons are not yet clear, but it may be due to the lack of the pelletization step, and the use of the stable precursors.

Hydrothermal Method

Hydrothermal synthesis is performed at relatively low temperatures, thus, it is a low cost and energy-saving method to prepare fine particles. Lee et al. (2008) reported the preparation of layered Li_0.88[Li_0.18Co_0.33Mn_0.49]O₂ nanowires via hydrothermal processing at 200°C. As prepared K_0.3MnO₂ is dispersed in a solution of Co(NO₃)₂ in distilled water, then kept in an autoclave at 200°C for 5 days, and washed thoroughly with water to remove residues and dissolved potassium ions. The obtained powders are Co_0.4Mn_0.6O₂ nanowires. Finally, Co_0.4Mn_0.6O₂ nanowires are mixed with LiNO₃·H₂O (wt/wt ratio is 4:1) in 100 ml of distilled water, and maintained in an autoclave at 200°C for 2 days. As-prepared powders are rinsed with water, and vacuum dried at 120°C. The materials exhibit excellent rate capability (220 mAh g⁻¹ at 1 C charge and 15 C discharge between 2 and 4.8 V), but poor cyclability (capacity retention of 92% after 50 cycles at 1 C) due to the high-specific areas of the nanowires, which reduce the diffusion path (increases the rate capacity) and increase the electrode/electrolyte contact area (accelerates side reactions, especially at high-charge potentials). In addition, the procedure is complicated and time consuming.

Wei et al. (2013) proposed a simpler hydrothermal route for preparation of Li[Li_0.2Co_0.4Mn_0.4]O₂. A suspension containing stoichiometric amounts of lithium, manganese, cobalt acetates is prepared, with oxalic acid as a precipitating agent and acetic acid as an additive. This suspension is heated in a Teflon container at 150°C for 3 h, then dried, pelletized, preheated at 450–500°C, and finally calcined at 750°C in air. The cathode material exhibits relatively low-specific discharge capacity of 180 mAh g⁻¹ between 2 and 4.6 V vs. Li⁺/Li° at 100 mA g⁻¹. Moreover, high-temperature heat treatment is another disadvantage of this preparation route.

Surface Modifications

Surface of high-energy materials can be modified with Al₂O₃, CeO₂, ZnO₂, or ZnO (3 wt% in the final product) (Wu and Manthiram, 2009), TiO₂ (Zheng et al., 2008), FePO₄ (Wang et al., 2013), or treated with 0.1 M HNO₃ (Kang et al., 2006). The surface modified cathodes with oxides (Wu and Manthiram, 2009) or FePO₄ (Wang et al., 2013) show higher first coulombic efficiency, discharge capacity, and capacity retention than the unmodified samples due to the retention of higher number of oxide ion vacancies in the lattice after the first charge (Wu and Manthiram, 2009; Wang et al., 2013). In addition, it was reported that TiO₂-coated Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ has enhanced thermal stability and cyclability (Zheng et al., 2008). The authors proposed that coating layer prevents the direct contact between active material and electrolyte, thus, reduces the side reactions between them at high-charge voltage region. On the other hand, it is suggested that acid treatment eliminates the first cycle irreversible capacity loss (first coulombic efficiency is close to 100%) by chemical activation of the Li₂MnO₃ phase. However, the acid treatment significantly reduces the cyclability and rate capability because of the H⁺-exchange process and water entrapment inside the cathode powder (Kang et al., 2006). Immersion of the xLi₂MnO₃·(1 − x)LiMO₂ powders in a Li–Ni–PO₄ solution using sol-gel method, followed by a heat treatment period, leads to the formation of new materials with enhanced rate capability (Kang and Thackeray, 2009). Such treatment creates a stable protective Li–Ni–PO₄ layer, which exhibits high-lithium-ion conductivity at high-working potentials (4.6 V vs. Li⁺/Li). Sun et al. (2012) reported that coating the high-energy cathode particles with AlF₃ could improve their electrochemical performance, both rate capability and cyclability. It is proposed by the authors that the Li₂MnO₃ phase is selectively converted to the spinel structure by chemical leaching of Li from Li₂MnO₃ due to the presence of the AlF₃ coating. Liu et al. (2010) introduced surface coating of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ cathodes by aluminum using a thermal evaporation technique. Aluminum coating increases the first discharge capacity and coulombic efficiency, improves the cyclability, and enhances the rate capability. The suppression of oxygen vacancy elimination at the end of first charge results in the increase in first discharge capacity, while the suppression of oxygen vacancy elimination and side reactions in the subsequent cycles leads to cyclability improvement. Rate capability enhancement can be explained by the increase in the surface conductivity. Zhao et al. (2011) reported that coating Li[Li_0.2Ni_0.2Mn_0.6]O₂ particles with manganese oxide (4 wt%) could significantly improve the cyclability and rate capability, although the reasons are not clear yet. Similar material was prepared by Wu et al. (2012), which exhibits lower charge transfer resistance in comparison with the uncoated one. In the study of Kang et al. (2005), Li[Li_0.2Ni_0.2Mn_0.6]O₂ is coated with amorphous Al(OH)₃ created from the hydrolysis reaction of Al(C₃H₇O)₃. The Al(OH)₃ coated cathode has a lower charge transfer resistance than the uncoated one, thus, has better rate capability. Moreover, the Al(OH)₃ coating also improves the thermal stability of the cathode powders.

Recently, Wu et al. (2014) proposed the coating of Li_1.2 Mn_0.6Ni_0.2O₂ by an ultrathin Li_1+xMn₂O₄ layer. Polyvinylpyrrolidone is simply dispersed on the Li_1.2Mn_0.6Ni_0.2O₂ materials, and the mixture is heat treated at 750°C to form an ultrathin Li_1+xMn₂O₄ membrane on the surface (Figure 3). This high-lithium conductive membrane promotes the lithium transportation between electrolyte and the layered active material, and stabilizes the layered bulk during high-voltage cycling. As a result, rate capability, cyclability, and thermal stability are all improved.

FIGURE 3

Figure 3. HRTEM images of the pristine layered lithium-rich materials (PLLR) sample (A) and the ultrathin spinel membrane encapsulated- layered lithium-rich cathode (USMLLR) sample (B). The inset is two times enlarged view in green rectangle of (B). XRD patterns (using Cu Kα radiation) (C) and Raman spectroscopies (D) of the PLLR sample and the USMLLR sample. Reproduced with permission from Wu et al. (2014).

Conclusion

Successful preparation of xLi₂MnO₃·(1 − x)LiMO₂ cathode materials that exhibit a specific discharge capacity of 200 mAh g⁻¹ or higher at 0.1 C rate between a potential range of 2–4.8 V vs. Li⁺/Li is crucial to the development of high-energy lithium-ion batteries. There may be a number of ways to achieve the challenge in research laboratories. From an engineering point of view, choosing a feasible preparation method for straightforward commercialization is the challenge since most of the current state-of-the-art methods require significant heating and long-processing times. Many research groups follow the initiation of researchers at the Argonne National Laboratory to prepare the cathode materials by the combination of co-precipitation and solid-state reactions. However, complicated procedures, long-heat treatment times and high-heat treatment temperatures are current drawbacks, which cloud the competitiveness of the high-energy cathode technology. It is believed that solid-state reaction, combustion, sol-gel, and co-precipitation methods will be further optimized and modified to deliver more promising battery performance (both specific capacity and cyclability), together with shortening the preparation times, reduction of heat treatment durations, and temperatures.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was financially supported by Positec, Natural Sciences and Engineering Research Council of Canada (NSERC), Canadian Foundation for Innovation (CFI), and the Canada Research Chairs (CRC) program. The authors would like to thank Dr. J. Byerley for assistance in editing the manuscript.

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