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Li7La3Zr2O12 (LLZO): An Overview
Li7La3Zr2O12 (LLZO) is an inorganic garnet type solid electrolyte which has proven to be one of the most promising electrolytes because of its high ionic conductivity at room temperature, low activation energy, good chemical and electrochemical stability, and wide potential window.
Among many energy storage devices, lithium-ion battery has been the most commonly used technology because of its advantages including high energy density, long duty cycle, low self-discharge, and light weight . However, there are some concerns about lithium-ion batteries like: safety issues because of the formation of dendrites, reduced power capability because of the formation of solid electrolyte interface (SEI) from continuous charging and discharging, low ionic conductivity, and high cost compared to other existing rechargeable batteries. Extensive research is being carried out by many groups to resolve these issues aiming to improve the performance of lithium-ion batteries.
Electrolyte is one of the most important parts that determine the performance of lithium-ion batteries. According to the state of form, electrolyte materials can be categorized as liquid electrolytes, polymer electrolytes, ionic electrolytes, and solid electrolytes.
2. Structural Analysis of LLZO
3. Synthesis Techniques of LLZO (Li7La3Zr2O12)
The first LLZO was prepared by Murugan et al. in 2007 using solid state reaction technique where they used sintering temperature of 1230 °C for 36 h . After 2007 this became the most widely used method; however, some researchers reported some concerns regarding this process: (1) the method is difficult in achieving desired stoichiometric structure, (2) high sintering temperature introduces impure phases into the system, (3) the long period of high sintering processing causes too much Li loss from the system, and (4) at high temperature, other foreign elements are introduced from the crucible containing the sample .
To avoid the problems arising from high sintering temperatures during solid state reaction, which causes Li loss and creates impure phase formation in the system, the sol-gel method has been developed which involves hydrolysis and polymerization. The sol-gel method uses relatively low temperatures and yields uniformly distributed finer sized particles without needing intermittent grinding .
The Pechini method is a modified technique of the sol-gel method which involves the ability to chelate metal ions with the help of carboxylic acids such as tartaric acid, citric acid, etc. and forms a polymeric resin due to polysensitization when heated with polyethylene glycol or another hydroxylic alcohol agent. This process requires lower sintering temperature, shorter reaction time and yields uniformly distributed nanosized particles. Using the Pechini method, Gao et al. synthesized Li 5La 3Bi 2O 12 nanocrystalline powder , and Jin et al. synthesized stable cubic Al-doped LLZO .
Pulsed laser deposition (PLD) has also been used for the fabrication of LLZO. In PLD, a laser beam usually ablates the targets at a suitable angle which helps to preserve the stoichiometry and composition of the target. PLD is very flexible process as it allows the adjustment of conditions that are appropriate for deposition such as the energy of laser density, temperature, and pressure. Also, it reduces the Li loss that occurs during the synthesis process as it allows films to be grown at low temperature in a shorter period. However, a very few studies have been reported so far that used PLD for LLZO preparation. The effect of different substrates on the growth of LLZO at different deposition temperature were investigated by Park et al. . Tan et al. used this method to prepare LLZO on sapphire and SrTiO 3 substrates .
4. Sintering Techniques of LLZO
Sintering is one of the most important steps during the fabrication of solid-state electrolytes as it plays a vital role in densifying the pellet of electrolyte. Dense pellets have many advantages such as: (1) decreasing grain boundary resistance and thus decreasing the activation energy to increase the Li ionic conductivity, (2) suppressing the formation of dendrites and accordingly increasing the safe operation of the battery, and (3) increasing the mechanical strength of the battery. Being not dense enough or possessing high porosity in the solid electrolyte increases the chance of fragility, and hence may cause mechanical failure of the device. There are some techniques like furnace sintering , hot pressing , field assisted sintering technology (FAST)  and spark plasma sintering (SPS)  reported in the literature that can be used to improve the density of pellets.
Hot pressing involves heat and pressure, where samples are sintered at high temperatures under a certain pressure. Simultaneous application of high temperature and pressure used in this process helps to stabilize the structure and densifies the pellets better. This method is also known as hot isostatic pressing (HIP).
Field-assisted sintering technology (FAST) is a sophisticated modern technology used to sinter samples at a high heating rate. The process of FAST is similar to that of hot isostatic pressing (HIP), however, a dc current passes through the tool along with the uniaxial pressure to assist in sintering process. In other words, the additional heat is generated by an external electrical circuit. FAST has many advantages over other sintering methods in terms of the fast-sintering process that saves time and reduction in Li ion loss that usually occurs due to a long sintering duration. As the synthesis of raw materials and sintering are done together at one time, it avoids the process of tedious intermittent grinding and repeated long-time sintering.
The entry is from 10.3390/electrochem2030026
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