Surface Area Increase of Silicon Alloys in Li-Ion Full Cells Measured by Isothermal Heat Flow Calorimetry

Abstract

Li-ion pouch cells utilizing a negative electrode formulated with 15 wt% of an engineered Si alloy in a graphite composite electrode were cycled in an isothermal heat flow calorimeter against a LiCoO 2 positive electrode. Two different electrolytes were investigated: a blend of ethylene carbonate and ethyl methyl carbonate (3EC:7EMC) and a blend of ethylene carbonate, ethyl methyl carbonate and 1-fluoro ethylene carbonate (27EC:63EMC:10FEC). Both electrolytes were 1 M in LiPF 6 salt. The parasitic thermal power and coulombic efficiency was derived from isothermal heat flow measurements and high precision current-source meters. Cells without FEC showed high parasitic thermal power which increased with cycle number indicative of a surface area increase which was confirmed by post-cycling scanning electron micrographs and surface area measurements. Cells with FEC showed relatively stable parasitic thermal power. These measurements demonstrate the surprising function of FEC in controlling or attenuating the evolution of surface area in Si alloys. Vinylene carbonate was also found to be effective at controlling the increase in alloy surface area. Meeting the promise of silicon or Si alloys to increase the energy density of Li-Ion cells has been the subject of a great deal of research and development. To date the full commercial impact of this materials technology has not been met owing to the difficulties found with silicon. [1][2][3][4][5][6] In general the high capacity fade often found in Li-ion cells containing Si has 3 main causes: 1) mechanical or electrical disconnect of the alloy particles in the composite electrode owing to large volume changes 1-4 2) crystallization effects, mainly the formation of the Li 15 Si 4 phase, which shows high irreversibility 5-7 and 3) unusual electrolyte reactivity. 3,6,8,9 It is the latter failure mechanism which this paper addresses.It has been recognized and shown by many that the electrolyte composition can be crucial for cycling capacity in cells containing silicon or Si alloys.9,10 Recent work by Petibon et al. on pouch cells using a negative electrode composed of 72.3 wt% graphite, 15 wt% Si alloy, 10 wt% KS6 and 2.7 wt% carboxymethyl cellulose-butadiene styrene binder (CMC:SBR) showed approximately 300 cycles to 80% of the original capacity at a C/2 rate. In that work the base electrolyte was 1 M LiPF 6 in a blend of EC, EMC and FEC in the weight ratios of 27:63:10 respectively. The capacity fade was linear until about 250 cycles at which point the capacity fade rate accelerated significantly. 4 The authors showed that the additive FEC was consumed during cycling and the acceleration of the fade rate occurred when the FEC was consumed. Analysis of the gas produced during the first cycle was shown to be mainly CO 2 with small amounts of H 2 and CO. The authors attributed the gas formed as products from the reductive formation of SEI layers at the graphite and alloy surfaces. They argued that the absence of ethylene suggests that FEC inhibits the reduction of EC and noted ...

Robotic cell assembly to accelerate battery research

“…Since the bimodal curve shape distribution is visible on both the charge and discharge data, we believe that these minute variations highlight the necessity to produce larger arrays of cells as they would have otherwise likely not been captured. Comparing the herein acquired data to that found in literature on LNO the region showing increased variance that is denoted by the roman numeral I in Figure 4 could be attributed to the H1 and H1 to H2 phase transition or more likely to the insertion into Si in the Si-C anode 22 . Since there is some considerable variance in Si-particle loading (see SI) across the anode sheets we believe this variance to be originating from the electrodes and not necessarily from assembly or randomness.…”

The Effect of Carbon Dioxide on the Cycle Life and Electrolyte Stability of Li-Ion Full Cells Containing Silicon Alloy

“…3 Figures 2a and 2b show the parasitic thermal power and coulombic inefficiency (CIE = 1-CE) of a cell containing CO 2 charged by the dry ice method and a cell with 10 wt% FEC in the base electrolyte. These cells were cycled 50x and 42x respectively before they were inserted into the calorimeter.…”

“…While there can be several reasons for the inferior cycle life of silicon and silicon alloys one important contributor to poor cycle life is the erosion of the silicon particle by parasitic electrolyte reactions. [1][2][3] Because of the volume changes Si experiences during cycling, continuous passivation is required at the surface and the demands on the electrolyte system are greater than for graphite.Carbon dioxide (CO 2 ) has been described as an effective electrolyte additive in metallic Li cells many years ago. 4 In that work it was shown that cycling efficiencies of the Li electrode were greatly increased in the presence of CO 2 .…”

Insights into Reactivity of Silicon Negative Electrodes: Analysis Using Isothermal Microcalorimetry

“…31 IMC has been used to study the role of carbon dioxide on the formation of SEI with a silicon anode. 32 Further, IMC was previously utilized to study battery systems containing 15 wt % engineered Si alloy 33 and 20 wt % nano silicon composite anodes in full and symmetric cells, respectively. 34 In the prior reports, the parasitic heat evolution was probed at high cycle number where the entropic contributions to heat flow were neglected, as entropy changes during discharge and charge were assumed to be reversible.…”