All-Solid-State Lithium Battery Using Transparent Garnet-Type Al-Doped Li7La3Zr2O12
Authors: Sugata Shoichi , Saito Noriko , Watanabe Ken , Ohnishi Tsuyoshi , Kim Je-Deok , Honma Itaru 
Published: February 2020

All-solid-state Li ion batteries using solid electrolytes have safety advantages such as nonleakage or nonflammability of liquid electrolyte. Li7La3Zr2O12 (LLZ) is one of the promising electrolytes for all-solid-state battery due to its high Li ion conductivity and stability against Li metal anode. However, the power density of the battery using LLZ electrolyte is insufficient for practical use because of the problem of lithium dendrite growth at high current density. To suppress lithium dendrite growth, the densification of LLZ electrolyte is one of the effective approaches. Another problem is the high resistivity of the electrode/electrolyte interface in a solid-state battery. It is difficult to form a good ion conducting path at solid/solid interface between electrode and electrolyte especially for the bulk type solid-state battery. We therefore expect that the interface resistance can be decreased by using soft material as cathode. In this research, LLZ electrolyte with high density was prepared by Hot Isostatic Pressing (HIP) treatment, and Li ion battery was fabricated by using LLZ electrolyte and adding conductive ionic liquid to the cathode. LLZ samples were synthesized by the Pechini method. LiNO3 anhydrous, La(NO3)3·6H2O, ZrOCl2·8H2O, and Al(NO3)3·9H2O were used to prepare nitrate precursor solutions. 2, 5 and 10% Excess Li and 1wt% of Al were added to the nitrate solution. Citric acid and ethylene glycol were added to the nitrate solution as complexing agents. The mixture was heated at 130°C until a solidified precursor formed. The precursor was calcined at 700°C for 5 h. After the calcination, the powder was pressed into a pellet and the pellet was sintered at 1200°C under O2 atmosphere. The sintered LLZ pellets were HIP treated under the conditions of 1180°C, 132 MPa and a time of 2 hours. Li ion conductivity of LLZ sample was examined by the AC impedance method. Au was sputtered on both sides of the pellet to ensure electrical contact. For the fabrication of Li battery, a composite cathode was prepared by mixing LiFePO4(LFP) with carbon black, and then ionic liquid was added. For the preparation of ionic liquid, 1 M lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) was dissolved in the above 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI). Li battery with LFP/LLZ/Li configuration was fabricated by putting the composite cathode and Li metal on each side. The electrochemical properties of the LFP/LLZ/Li cell were investigated by cyclic voltammetry and galvanostatic measurement.

 The samples which include 2 or 5% of Li excess in raw materials were transparent and had a density of >99%. LLZ sample which includes 2% of Li excess in raw materials showed a conductivity of 1.2 × 10−4 S cm−1 at 30°C. The conductivity was about the same with previously reported results. The conductivity did not deteriorate, and only the density improved by HIP treatment. The sample after HIP treatment showed a higher conductivity than that before HIP treatment over the entire temperature range. Using HIP treatment for LLZ sample, we have shown that the conductivity was improved by the densification of LLZ sample. Figure 1 shows galvanostatic charge and discharge curves of the LFP/LLZ/Li cell at the current density of 0.046 mA g-1 (0.046 C rate). The specific charge capacity of the cathode active material was 168 mAh g-1 and the discharge capacity was 169 mAh g-1. LFP has a theoretical capacity of 170 mAh g-1, and the composite cathode achieved almost full capacity of LFP. The coulomb efficiency was approximately one, and any side reactions were not observed. The magnitude and the shape of the discharge curves did not change after the fifth cycle. The LFP/LLZ/Li cell showed high stability against Li metal anode and good cycle stability.

Figure 1