All-solid-state Li battery using a light-weight solid electrolyte

Transcription

1 All-solid-state Li battery using a light-weight solid electrolyte Hitoshi Takamura Department of Materials Science, Graduate School of Engineering, Tohoku University Europe-Japan Symposium, Electrical Technologies for the Aviation of the Future, Delegation of the Europe Union to Japan, 1 26 MAR 2015

2 Contents Introduction LiBH 4 as an ionic conductor Fabrication & properties of all-solid-state Li battery using LiBH 4 Bulk type Thin-film type with an intermediate layer Li precipitation behavior in a LiBH 4 -Cu composites 2

3 All-solid-state Li secondary battery Table: Li-ion conductors to be used for all-solid-state LIB Advantages: Low risk of fire No liq. leakage Wide operating Temp. No Li dendrite growth t Li 1 Flexibility of cell design Drawbacks: Large G.B. resistance Low power density Interfacial resistance between electrolytes and Li New Li-ion conductor: LiBH 4 3

4 Solid electrolyte: LiBH 4 H.T. phase L.T. phase Orthorhombic Hexagonal [1] M. Matsuo et al., Appl. Phys. Lett. 91 (2007) [2] H. Maekawa et al. J. Am. Chem. Soc. 131 (2009) 894 Enhancement of Li-ion conductivity at 115 C accompanied by phase transition The H.T. phase can be stabilized by partial substitution of BH 4 - by halides. 4

5 Solid electrolyte: LiBH 4 Chemical compatibility with Li Li LiBH High capacity 4 Li High power Next generation Li metal Li LiBH 4 High capacity Li (10 times higher than graphite) High electric conductivity Dendritic growth Highly reducing material Hz 136 Ω : 137 Ω Thermodynamically stable with Li metal No SEI Negligible Electrode & G.B resistance 5 :

6 Solid electrolyte: LiBH 4 Advantages Light weight < 0.7 g/cm 3 Good compatibility with Li metal Plasticity High density Lower interfacial resistance Drawbacks Easy to oxidize 10 μm SEM image of LiBH 4 prepared by cold pressing (1t cm -2 ) It may cause a redox reaction with cathode materials, e.g. LiCoO 2 Unstable in air Difficult to handle Reaction with water Anode LiBH 4 Cathode LiBH 4 + 2H 2 O LiBO 2 + 4H 2 6

7 Key issues to use LiBH 4 in ASS Li batteries To manage interface between the LiBH 4 electrolyte and LiCoO 2 cathode. To use LiBH 4 for anode side to realize a highcapacity anode. 7

8 Result 1 Electrochemical properties of a simple cell Li LiBH 4 LiCoO 2 8

9 Charge-discharge curves for Li LiBH 4 LiCoO 2 Charge-discharge Nyquist plot R bulk R ct Rapid capacity fade Large overvoltage Significant degradation The large interfacial resistance (350 Ω) at 1 st cycle further increases with charge-discharge cycles. 9

10 Interface of LiBH4 and LiCoO2 Before charge-discharge After charge-discharge 10 μm 10 μm Black area corresponds to LiBH4. Some holes were observed in LiBH4 after charge-discharge Gas (H2) evolution 10

11 Raman spectra of the LiCoO 2 cathode LiCoO 2 LiCoO 2 Co 3 O 4 CoO(OH) Decomposition reaction Li 0.5 CoO LiBH 4! 1 4 Co 3O CoO(OH)+1 8 LiBO Li 2O H 2 Charged state 11 Not identified

12 Result 2 Suppression of the interfacial resistance by use of an intermediate layer 12

13 Intermediate layer As for the high interfacial resistance between LiCoO 2 cathode and electrolyte, that is also an issue to overcome in sulfide and oxide electrolytes. It is considered to be due to space charge layer or mutual diffusion. An intermediate layer deposited on LiCoO 2 surface is reportedly effective to suppress the interfacial resistance. Table. Previous reports for the intermediate layer on a LiCoO 2 electrode Materials, thickness Electrolyte Effect Li 4 Ti 5 O nm Li 3.25 Ge 0.25 P 0.75 S 4 R interface 1000 Ω 40 Ω [1] LiTaO nm Li 3.25 Ge 0.25 P 0.75 S 4 R interface 1000 Ω 12 Ω [1] LiNbO 3 Li 10 GeP 2 S 12 Suppression of R interface [2] SiO 2 2 nm Li 2 S P 2 S 5 glass-ceramics R interface 270 Ω 220 Ω [3] Li 2 SiO 3 2 nm Li 2 S P 2 S 5 glass-ceramics R interface 270 Ω 130 Ω [3] Li-Nb-O Li 7 La 3 Zr 2 O 12 R interface 2700 Ω 150 Ω [4] ZrO 2 < 10 nm Organic liquid Cycle performance [5] [1] Y. Seino et al., SSI 176 (2005) [2] R. Kanno et al., J. Electrochem. Soc. 148 (2001) A742. [3] A. Sakuda et al., J. Electrochem. Soc. 156 (2009) A27. [4] 8 th Solid Ionics Seminar, abstract [5] Y. J. Kim et al., J. Electrochem. Soc. 150 (2003) A

14 Cross-sectional SEM image of the thin-film 25 nm Li 3 PO 4 -coated LiCoO 2 P Co EDX analysis 25 nm-thick Li 3 PO 4 intermediate layer was grown on the columnar LiCoO 2 thin-film. 14 Takahashi et al., J. Power Sources, 226 (2013) 61.

15 Electrochemical properties Charge-discharge Nyquist plot 1st charge 30th charge 97 % of the initial capacity was retained after 30 cycles The value of the interfacial resistance (21 Ω) was 1/1000 of that in a cell without the intermediate layer. Cathode LiCoO 2 bulk LiCoO 2 film Li 3 PO 4 /LiCoO 2 film R interface 350 Ω 15 kω 21 Ω 15 Takahashi et al., J. Power Sources, 226 (2013) 61.

16 Raman spectra Uncoated LiCoO 2 (powder) Li 3 PO 4 -coated LiCoO 2 thin-film LiCoO 2 Co 3 O 4 CoO(OH) LiCoO 2 Co 3 O 4 Co 3 O 4 in the thin-film was formed in the PLD deposition. The Li 3 PO 4 intermediate layer suppressed the chemical reaction between LiBH 4 and LiCoO Takahashi et al., J. Power Sources, 226 (2013) 61.

17 Other intermediate layers Uncoated LiCoO2 thin-film 2 µm Ra = 2.0 nm 25 nm Li3PO4-coated 15 nm LiNbO3-coated 2 µm 2 µm Ra = 7.3 nm Ra = 6.3 nm 25 nm Al2O3-coated 2 µm Smooth surface was obtained for LiNbO3 and Al2O3 Several Islands (t 100 nm) was observed for Li3PO4. 17 Takahashi et al., Solid State Ionics, 262 (2014) 179.

18 Optimization of the intermediate layer 18 Takahashi et al., Solid State Ionics, 262 (2014) 179.

19 Summary All-solid-state lithium battery using LiBH 4 as an electrolyte was fabricated: LiBH 4 having excellent Li-ion conductivity, chemical stability and plasticity works as a novel solid electrolyte for all-solid- state Li 10 μm battery. Intermediate layer is effective to suppress the interfacial reaction and to improve the capacity retention up to 97% after 30 cycles. A concept of composite anode using LiBH 4 and Cu was examined; the connection of plating Li inside the electrolyte seems to cause the degradation. The mixture of comparable size of electrolyte and electrode is preferred to improve coulomb efficiency. 19