Katsuya Hayashi

A consideration of the morphology of electrochemically deposited lithium in an organic electrolyte

Abstract Lithium rechargeable cells with lithium metal anodes are widely considered to have the highest energy density among comparable cells. However the life cycle and thermal stability of these cells must be improved. The poor performance of lithium metal cells is mainly explained by lithium dendrite growth. With a view to overcoming this problem, we considered the lithium deposition mechanism. We have been carrying out various experiments on the lithium deposition behavior. In this paper we used these results to propose the current and most likely lithium deposition mechanism. We suggest that lithium dendrites may be called whiskers because their shape satisfies the definition of whiskers as `fibrous crystals. Their tip morphology remains unchanged during their growth, which means they grow from the base in the same way as whiskers of tin from thin films under stress. Lithium deposited under a protective film will experience stress because the deposition is non-uniform. The protective film will break in order to release this stress thus, lithium whiskers may then grow in the form of extrusions. To support our assumption, we calculated the possible morphology of the lithium with the boundary condition that pressure induced by the surface tension is the same throughout the lithium surface. The calculation indicated three types of shape depending on the value of the surface tension and internal pressure. If lithium deformation is limited by the creep strength of bulk lithium and the lithium whiskers, the whisker growth is described by the calculated shape.

Influence of electrolyte composition of ethylene carbonate/propylene carbonate mixed systems on the properties of rechargeable Li/amorphous V2O5P2O5 cells

This paper examines the influence of electrolyte composition (solvent mixing ratio) of LiAsF6-ethylene carbonate (EC)propylene carbonate (PC) mixed solvent electrolytes on the rate capability and the charge-discharge cycling life of AA-type lithium (Li) metal/amorphous (a-)V2O5ue5f8P2O5 (95:5 in molar ratio) cells. In addition, fundamental abuse tests were carried out on the Lia-V2O5ue5f8P2O5 cells consisting of a heating test at 130 °C and an external short circuit test at 25 °C. As a result, the following three facts were determined. First, the rate capability does not differ greatly with a change in electrolyte composition. Second, the cycle life reaches its maximum value at an ECPC volume mixing ratio of 1:9, although the Li cycling efficiency obtained from the Li half-cell test increases with an increase in EC content in the ECPC mixed system. This is the result of an interaction between the cathode and electrolyte leading to the formation of film containing vanadium on the Li anode surface different from that usually formed in Li metal batteries by the reaction between the electrolyte and Li. Third, fundamental cell safety is ensured by the above abuse tests. The relatively better results of the heating tests for the Lia-V2O5ue5f8P2O5 cells are because of the enhancement of the thermal stability of the Li anode with the special surface film in the Lia-V2O5ue5f8P2O5 cell system.

Electrolyte for high voltage Li/LiMn1.9Co0.1O4 cells

Abstract An electrolyte for high voltage lithium metal anode cells must simultaneously satisfy at least the following requirements; (i) high cycling efficiency on the lithium metal anode; (ii) higher oxidation potential than the charging voltage, and (iii) high specific conductivity. We have examined various electrolytes for lithium metal anode cells using a high voltage cathode, LiMn 1.9 Co 0.1 O 4 . Of the electrolytes resistant to high voltage that we used, a system containing 60 to 90 vol.% of dimethyl carbonate (DMC) mixed with ethylene carbonate (EC) and 1.0 M lithium hexafluorophosphate (LiPF 6 ) provided the best cycling efficiency on a lithium metal anode, as well as a high specific conductivity around 10 mS cm −1 at 20 °C.

Ethylene carbonate/propylene carbonate/2-methyl-tetrahydrofuran ternary mixed solvent electrolyte for rechargeable lithium/amorphous V2O5-P2O5 cells

We have been studying a cell system which consists of an amorphous (a-) V2O5-P2O5 (95:5 in molar ratio) cathode, a lithium (Li) metal anode and an organic electrolyte by fabricating an AA-size prototype cell. In a previous study, we reported the influence of the mixing ratio of ethylene carbonate (EC) and propylene carbonate (PC) solvents, in EC/PC binary mixed solvent systems incorporating LiAsF6 as the solute, on the cell performance and safety of Li/a-V2O5-P2O5 cells. In this paper, we examine the influence of LiAsF6-EC/PC/2-methyl-tetrahydrofuran (2MeTHF) (solvent mixing volume ratio; 15:70:15) ternary mixed solvent electrolyte on the properties of AA-size Li/a-V2O5-P2O5 cells. The goals are to improve the cycle life of Li/a-V2O5-P2O5 cells while ensuring their safety and controlling their cost. Cell performance includes rate capability and charge-discharge cycle life. The abuse tests carried out on the Li/a-V2O5-P2O5 cells consist of a heating test at 130 °C and an external short circuit test at 25 °C. Cells with a ternary mixed solvent electrolyte have a longer cycle life than cells with EC/PC binary mixed solvent electrolytes. In addition, these cells are much less likely to suffer from soft shorting at low discharge rate cycles than cells with EC/PC binary mixed solvent electrolytes. This improvement in the cycle life is thought to result from an improvement in the Li anode cycling efficiency based on Li-half cell cycling test results. In addition, the abuse tests ensure the fundamental safety of cells containing ternary mixed solvent electrolyte.