Improvement in the Cycling Efficiency of a Lithium Anode

An electrochemical quartz crystal microbalance (EQCM) or quartz crystal mi-crobalance (QCM) can be used to estimate the surface roughness of deposited bthium [43], 

There are four possible ways of explaining [45] why a higher current discharge creates a smaller amount of dead lithium. 

2) When the discharge current is large, delocalized pits formed in the anode are shallow, so the deposited lithium whiskers can easily emerge from the pits, and stack pressure can be applied to them, as mentioned in Section 13.7.3. 

3) Isolated lithium near the anode becomes a local cell because of stray current. As the stray current is high when the cell discharge current is high, bthium recombination occurs easily at a high discharge current [46]. 

4) When the discharge current is high, transport of lithium ions becomes difficult and stripping occurs from the particle-like lithium on the tip and on the kinks of the fiber-like lithium. In this case, the fiber-like lithium rarely breaks and the efficiency increases. 

There have been many attempts to improve the cycling efficiency of lithium anodes. We describe some of them below, by discussing electrolytes, electrolyte additives, the stack pressure on the electrode, composite anodes, and alternatives to the lithium-metal anode anode. 

Hayashi et al. [50] investigated the electrolyte materials and their compositions with various carbonates and ethers as 

3-Propylsydnone (3-PSD) was proposed as a new solvent by Sasaki et al. [51]. The cycling efficiency of lithium on a Ni electrode of the ternary mixed-solvent electrolyte of 3-PSD, 2MeTHF and 2, 5-dimethyltetrahydrofuran with LiPF6 was about 60 percent, and it was stable with cycling. 

2MeTHF to EC/PC causes the soft shorting to decrease dramatically. 

Another influence that electrolyte materials have on the cycle life of a practical lithium cell results from the evolution of gas as a result of solvent reduction by lithium. For example, EC and PC give rise to [53] evolution of ethylene and propylene gas, respectively. In a practical sealed-structure cell, the existence of gas causes irregular lithium deposition. This is because the gas acts as an electronic insulator and lithium is not deposited on an anode surface where gas has been absorbed. As a result, the lithium cycling efficiency is reduced and shunting occurs. 

An ether, such as 2MeTHF, has the effect of raising the FOM. When an A A Li/a-V2O5-P2O5 cell with an LiAsF, -EC/PC electrolyte is cycled with a low discharge current of 60 mA (0.1 C discharge rate), the cell shows a shunting tendency (a partial internal short) near the end of its cycle life [52]. However, the addition of 

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A lithium anode mixed with conductive particles of Cu or Ni was studied by Saito et al. they obtained an improvement in the cycling efficiency (Fig.6) [80]. Their idea is based on the recombination of dead lithium and formation of many active sites for deposition. 

Many studies have been undertaken with a view to improving lithium anode performance to obtain a practical cell. This section will describe recent progress in the study of lithium-metal anodes and the cells. Sections 3.2 to 3.7 describe studies on the surface of uncycled lithium and of lithium coupled with electrolytes, methods for measuring the cycling efficiency of lithium, the morphology of deposited lithium, the mechanism of lithium deposition and dissolution, the amount of dead lithium, the improvement of cycling efficiency, and alternatives to the lithium-metal anode. Section 3.8 describes the safety of rechargeable lithium-metal cells. 

The VC was first explored as an electrolyte solvent,which affords a good electrolytic conductivity due to its extremely high relative permittivity (e = 127). However, it became a typical compound as an anode passivation film-forming agent, after it was found that the addition of a small amount of VC suppressed gas evolution during the initial charge with the enhanced cycle efficiency, and protected the decomposition of reduction-susceptible solvents such as trimethyl phosphate (TMP). The excellent stability of the passivating layeE was demonstrated by the fact that the addition of 1 wt% of VC in 1 M LiPE /EC + DMC + DEC (33 33 33 wt%) improved the cycle life of commercial lithium-ion polymer ceUs. ... 

We believe that the advantage of these composite anodes is that they result in a uniform lithium deposition at the boundaries of two components that may improve the cycling efficiency. 

Arie et al. [116] investigated the electrochemical characteristics of phosphorus-and boron-doped silicon thin-film (n-type and p-type silicon) anodes integrated with a solid polymer electrolyte in lithium-polymer batteries. The doped silicon electrodes showed enhanced discharge capacity and coulombic efficiency over the un-doped silicon electrode, and the phosphorus-doped, n-type silicon electrode showed the most stable cyclic performance after 40 cycles with a reversible specific capacity of about 2,500 mAh/g. The improved electrochemical performance of the doped silicon electrode was mainly due to enhancement of its electrical and lithium-ion conductivities and stable SEI layer formation on the surface of the electrode. In the case of the un-doped silicon electrode, an unstable surface layer formed on the electrode surface, and the interfacial impedance was relatively high, resulting in high electrode polarization and poor cycling performance. 

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