Li Pre-Doping of Amorphous Si Thin Flakes in Li-Aromatic Complex Solutions

Li-Si alloy negative-electrodes have attracted much attention for use in lithium ion batteries with high energy density because of their high theoretical capacity (ca. 4200 mAh g^-1).  However, several problems, such as poor cycleability and low Coulombic efficiency particularly in the initial cycles, remain to be solved for practical use.  The poor cycleability is attributed to a large volumetric change during charge/discharge cycling.  Hence, we designed nano-flaked Si called Si Leaf-Powder^® (Si-LP), which can relieve the physical stress caused by expansion and shrinkage of Si during alloying/dealloying reactions with lithium.  We have reported that Si-LP with a thickness of 100 nm showed good cycleability.¹⁾

Another serious problem for Si negative-electrode is a large irreversible capacity (Q_irr) in the first cycle. Li pre-doping is a useful technique to cancel the large Q_irr.  We also reported that the large Q_irr (1000~1500 mAh g⁻¹) can be drastically reduced by Li pre-doping using a direct contact method with Li foil.²⁾ Unfortunately, however, several issues, such as control of Li-doping level and homogenous Li-doping, remained to be solved for practical use.

In this study, we developed a Li pre-doping technique for the amorphous Si-LP electrode using Li-naphthalene and Li-anthracene complex solutions.³⁾

The Si-LP composite electrode consisted of 83.3 wt.% Si-LP, 5.6 wt.% Ketjen black, and 11.1 wt.% carboxymethyl cellulose sodium salt. The Si-LP electrode was pre-doped using a coin-type two-electrode cell and Li foil as a lithium source and a reference electrode. The solutions were tetrahydrofuran (THF) containing naphthalene (NTL) or anthracene (ATC) of which the concentration ranged from 0.1 to 1.0 M.  The pre-doping was carried out by leaving the cell for a given time under the open-circuit condition.  The variation of open circuit voltage (OCV) with time was monitored.  After pre-doping, the cell was disassembled in an Ar-filled glove box and then the Si-LP electrode was rinsed with pure THF to remove the residual solution.

Charge and discharge properties of the pre-doped Si-LP electrodes were investigated using a coin-type two-electrode half-cell at a C/6 rate (1 C = 4200 mAh g^-1) at 30^oC. The working and counter electrodes were the pre-doped Si-LP electrode and a Li foil, respectively.  The electrolyte used was 1.0 M LiPF₆/EC+DEC (1:1 by vol.).

Figure 1 shows the variation of OCV of pre-doping cells with time.  The voltages descended to about 60 and 40 mV within 5 h after the start of pre-doping in 0.1 and 0.5 M Li-NTL solution, respectively. On the other hands, the voltage almost unchanged at around 240 mV during the first 10 h for Li-ATC, and then began to decrease gradually.

Table 1 shows charge capacity (Q_cha), discharge capacity (Q_dis) and pre-doped capacity (Q_pre) of the Si-LP electrodes in the 1st cycle in 1.0 M LiPF₆ / EC + DEC after pre-doping in Li-NTL or -ATC complex solution.  If the irreversible capacity is about 1200 mAh g^-1, which corresponds to the initial irreversible capacity of pristine Si-LP electrodes in 1.0 M LiPF₆/EC+DEC, Q_preis calculated from the following equation;

Q_pre = Q_dis – Q_cha + 1200 mAh g^-1

As a result, Q_pre was 2345 and 2429 mAh g^-1 for 1 h pre-doping in 0.5 M Li-NTL and 10 h pre-doping in 0.1 M Li-NTL solution, respectively. These values were close to typical reversible capacity of Si-LP (about 2300 mAh g^-1). These results suggest that the Si-LP electrodes were fully pre-doped in these conditions.

On the other hands, the Q_dis was almost the same as Q_cha, for 1 h pre-doping in 0.1 M Li-NTL solution, that is, the Q_pre compensated for Q_irr. The OCV at the end of pre-doping was 110 mV, as shown in Fig. 1. Very similar results were obtained for 22 h pre-doping in Li-ATC solution; Q_dis was close to Q_cha and the OCV reached 140 mV at the end of pre-doping. To clarify the meaning of the OCV, a pristine Si-LP electrode was charged at a constant voltage of 120 mV for 3 h, and then discharged.  The discharge capacity was evaluated to be 1500 mAh g^-1, which was almost equivalent to Q_irr of 1200 mAh g^-1. These results indicate that Li-doping level can be controlled by OCV.

References

1) M. Saito et al., Solid State Ionics, 225, 506 (2012).

2) T. Okubo et al., Solid State Ionics, 262, 39 (2014).

3) T. Tabuchi et al., J. Power Sources, 146, 507(2005).