What is a thin film solid electrolyte?

The conductivity of inorganic solid electrolytes is generally still significantly lower than that of liquid electrolytes and polymer electrolytes. Therefore, inorganic solid electrolytes are generally made into electrolyte films with a thickness of micrometers. They are used on microchips or used as power supplies for micromotors made with LIGA technology. superior. The more traditional method of preparing solid electrolyte film is to directly prepare the solid electrolyte on the conductive substrate or the surface of the electrode material by the sol-gel method.Using the sol-gel spin coating method, the thickness of about 0.10Al₂O₃·0.08Sc₂O₃·0.82ZrO₂ and 0.36μm 0.125Sc₂O₃·0.175TiO₂·0.7ZrO₂ solid electrolyte nanocrystalline film.. Experimental results show that both films begin to crystallize above 650°C. The higher the temperature, the more complete the crystallization, which can be completely crystallized at 800°C. The obtained nanocrystalline particles are in a pure cubic phase of fluorite structure, and the average sizes of the aluminum and titanium doped nanocrystalline particles are 47 nm and 51 nm, respectively. The aluminum-doped film is very uniform and dense, but there are a few micro-pores in the titanium-doped film. In order to avoid the formation of cracks in the preparation process of high temperature heat treatment, some people used polyvinylpyrrolidone as one of the components of the sol, and prepared Li_0.35La_0.55TiO₃ film by the sol-gel method. The room temperature ionic conductivity of the Li_0.35La_0.55TiO₃ film prepared at 1000°C can reach 10^-3S/cm. The LiNi_0.5Mn_1.5O₄ film prepared by the sol-gel method is used as the positive electrode of the battery, The contact resistance between the positive electrode membrane of the all-solid-state lithium-ion battery and the solid electrolyte membrane prepared with crystalline glass ceramic LiTi₂(PO₄)₃-AlPO₄(LTP) as the electrolyte is 90Ω/cm².

Some components that cannot be obtained by chemical equilibrium methods can be obtained by vacuum evaporation or sputtering methods. For example, amorphous LiAlF₄ films can be easily prepared by vacuum evaporation of equimolar LiF and AlF₃, but it is impossible to synthesize bulk materials because the compound will segregate in chemical equilibrium. It is Li₃AlF₆ and AlF₃. The conductivity of the thin film LiAIF₄ reaches 10^-4S/cm, which is much higher than that of LiF, AlF₃ or Li₃AlF₆.

Li₃(PO₄)-_xN_x(LiPON) has become the best solid electrolyte for all solid-state lithium-ion batteries. In the method for preparing LiPON thin film, LiPON thin film can be deposited by radio frequency sputtering Li₃PO₄ target under nitrogen or helium/nitrogen mixed gas conditions. The conductivity of the film Li_0.29S_0.28O_0.35N_0.09 at room temperature reaches 2×10^-5S/cm. The microstructure analysis of the LiPON film shows that when the film is formed in a nitrogen atmosphere, the resulting film is amorphous. The electrochemical stability of the electrolyte reaches 5.5V (relative to Li/Li⁺).

Li₃(PO₄)-_xN_x(LiPON) solid electrolyte

Using the method of pulsed laser ablation of Li₃PO₄ target, a lithium phosphorus oxynitride (LiPON) ion conductor film was deposited on an aluminized glass substrate in a nitrogen environment as the electrolyte of all solid-state lithium ion batteries (Al/LiPON/Al). The room temperature lithium ion conductivity of the LiPON film is 1.4×10^-6 S/cm. The (Li_0.5La_0.5)TiO₃(LLTO) thin film electrolyte with a thickness of 400nm was prepared on a Pt/TiO₂/SiO₂/Si substrate at 500℃. The room temperature conductivity is about 1.1×10^-5S/cm. Even after annealing at 600°C, the film is still in an amorphous phase.

In recent years, many researchers have used thin-film electrolytes for thin-film all-solid-state lithium-ion batteries. For example, an amorphous solid electrolyte film (1-x)LiBO₂·xLi₂SO₄(LiBSO) (x=0.4~0.8) with a thickness of 1~2μm is prepared by radio frequency magnetron sputtering. The room temperature ionic conductivity of the electrolyte increases with the increase of x, and reaches the maximum value at x=0.7, which is about 2.5×10^-6S/cm. At higher values ​​of x (x>0.7), due to partial crystallization, the conductivity of the film begins to decrease. The electrolyte membrane is electrochemically stable when the lithium potential reaches 5.8V. The combination of LiBSO thin film electrolyte and Li/TiS₂ microbattery shows good cycle performance. The thin films with nominal components Li_1.2Mn₂O₄ and BPO₄·0.035Li₂O were prepared by electrostatic spraying (ESD) method for all-solid-state lithium-ion batteries. When studying the relationship between the morphology of the film and the deposition conditions, such as the solvent composition of the precursor solution and the substrate temperature, it is found that using a lower substrate temperature and/or a higher solvent boiling point is beneficial to obtain a dense film. . If you use 85% (volume fraction) of batyl carbitol and 15% (volume fraction) of ethanol as a solvent and deposit at 250°C, a porous network structure film can be obtained. After the amorphous or microcrystalline Li_1.2Mn₂O₄ film obtained in the temperature range of 250-400°C is annealed at 600°C or higher, the film crystallizes into a spinel structure. The glassy BPO₄·0.035Li₂O layer can fill the micropores of the porous Li_1.2Mn₂O₄ to become a dense intermediate electrolyte layer. Thin-film rocking chair batteries were prepared with Li_1.2Mn₂O₄/BPO₄·0.035Li₂O/Li_1.2Mn₂O₄/Al. After charging, the open circuit voltage of the battery is about 1.2V. The solid solution of lithium aluminum oxide (Li₄GeO₄) and lithium vanadium oxide (Li₃VO₄) (general formula is Li_3.6Ge_0.6V_0.4O₄) can also be used as an all-solid-state high-temperature lithium battery with solid electrolyte. Cyclic voltammetry experiments show that lithium can be reversibly deposited on platinum and gold working electrodes when the potential of the lithium-aluminum reference electrode is below 0.5V; the working voltage is about 4.5V, and the solid electrolyte is electrochemically stable to any oxidation process . The all-solid-state lithium battery made of lithium aluminum alloy as the negative electrode and chemical vapor deposited titanium disulfide film as the positive electrode has an open circuit voltage of about 2.1V at 300°C. The battery also has a very high rate capability, which can be as high as Discharge at a current density of 100mA/cm².

The ionic conductivity of the lithium phosphorous oxynitride (LiPON) electrolyte film at 300K is 6.0×10^-7S/cm, and the electrochemical window is 5.0V. In addition, the positive electrode film Ag_0.5V₂O₅ is prepared by PLD and assembled with metal lithium to form an all-solid film Lithium battery: Li/LiPON/Ag_0.5V₂O₅. The open circuit voltage of the battery is 3.0V, and when the current density is 14μA/cm², the first discharge capacity is 62nA·h/(cm²·μm). In 10 cycles, the average capacity attenuation of each cycle is only 0.2%. The cycle life can reach 550 times. Using lithium metal as the negative electrode, LiPON as the solid electrolyte, and Li_xCoO₂ as the positive electrode, an all-solid thin-film micro-lithium battery was prepared. The electrical behavior and impedance increase of the battery were evaluated in the temperature range from -50°C to 80°C. The size of the battery was approximately It is 2cm long, 1.5cm wide and 15μm thick. The battery has a rate capacity of about 400μA·h. The battery is cycled 100 times at room temperature at a rate of 0.25C. The charge-discharge cut-off voltages are 4.2V and 3.0V, respectively. In 100 cycles, the battery does not have any capacity degradation. The measured battery capacity reaches 400 μA·h, and the Coulomb efficiency is 1, indicating that there is no parasitic side reaction in the battery reaction, and the insertion and extraction of lithium are completely reversible. At room temperature, the capacity when discharged at 2.5C reaches 90% of the discharge capacity at 0.25C.

LiPON solid electrolyte

Storing the battery in an environment of 80°C can cause permanent damage to the battery, because even if it returns to room temperature, the performance of the battery cannot be restored to a normal state. The internal resistance of the battery is measured before and after the battery is stored at different temperatures. The high-frequency impedance of the battery (usually attributed to the electrolyte and other impedances connected in series with the electrolyte) decreases as the temperature decreases, and the interface impedance of the battery decreases as the temperature decreases. The temperature decreases and increases. In addition, the electrolyte impedance accounts for approximately 2% of the battery's total impedance. After cycling, the battery's impedance increases significantly.

Using aluminum as the negative electrode, and then sputtering and depositing LiPON film on it as the electrolyte can be made into an all-solid-state lithium-ion battery. The lithium ion conductivity of LiPON film reaches 10^-6S/cm, which is similar to that of bulk materials. The bulk high-voltage cathode material Li₂CoMn₃O₈ is first made by sintering method, and then the bulk material and 20% (mass fraction) LiNO₃ are evaporated by electron beam, and the film is prepared under the condition of 10^-5mPa oxygen partial pressure. The fabricated thin-film battery can work between 3~5V (relative to Al or LiAl electrode). The room temperature chemical diffusion coefficient of Li_2-xCoMn₃O₈ (x is 0.1~1.6) measured by GIT technology is 10^-13~10^-12cm²/s. The impedance spectroscopy study results of the full battery system show that the charge transfer impedance is 290Ω, the electric double layer capacitance is about 45~70μF, the counter electrode area is 6.7cm², and the chemical diffusion coefficient is 10^-12~10^-11cm²/s.

A solid-state thin-film battery is assembled from amorphous Li₂O-V₂O₅-SiO₂ as the solid electrolyte (LVSO), crystalline LiCoO₂ as the positive electrode and amorphous SnO as the negative electrode. The composition analysis shows that the solid electrolyte composition is Li_2.2V_0.54Si_0.46O_3.4. Estimated from impedance spectroscopy, the ionic conductivity of the LVSO film at 25°C is 2.5×10^-7S/cm, and the activation energy is 0.54eV. The thin film battery can be operated in the air before charging. The open circuit voltage of the battery is 2.7V when it is fully charged, and it shows good reversibility after 100 cycles of 0~3.3V.

Amorphous Li₂O-V₂O₅-SiO₂ solid electrolyte

Inorganic solid electrolytes are mainly used in thin-film lithium batteries or lithium ion batteries, and inorganic solid electrolytes are promising in improving the safety and cycling of lithium ion batteries. An important reason why rechargeable lithium batteries have not been commercialized so far is that the SEI film formed on the metal lithium negative electrode due to electrolyte decomposition is unstable, resulting in poor cycle performance of the battery, and it is difficult to inhibit the formation of lithium dendrites.