Layered Oxide, Graphite and Silicon-Graphite Electrodes for Lithium-Ion Cells: Effect of Electrolyte Composition and Cycling Windows

102

135

Silicon-graphite electrodes usually exhibit improved cycling stability when limiting the capacity exchanged by the silicon particles per cycle. Yet, the influence of the upper and the lower cutoff potential was repeatedly shown to differ significantly. In the present study, we address this discrepancy by investigating two distinct degradation phenomena occurring in silicon-graphite electrodes, namely (i) the roughening of the silicon particles upon repeated (de-)lithiation which leads to increased irreversible capacity losses, and (ii) the decay in the reversible capacity which mainly originates from increased electronic interparticle resistances between the silicon particles. First, we investigate the cycling stability and polarization of the silicon-graphite electrodes in dependence on different cutoff potentials using pseudo full-cells with capacitively oversized LiFePO 4 cathodes. Further, we characterize postmortem the morphological changes of the silicon nanoparticles by means of scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) as a function of the cycle number. To evaluate the degradation of the entire electrode coating, we finally complement our investigation by impedance spectroscopy (EIS) with a gold-wire micro-reference electrode and post-mortem analyses of the electrode structure and coating thickness by cross-sectional SEM. Silicon is among the most promising anode materials for future lithium-ion batteries.1,2 For example, a prismatic hard case cell comprising a silicon-carbon anode with 1000 mAh gand an NMC811 cathode would offer a specific energy of up to ∼280 Wh kgcell . 3 In contrast to state-of-the-art graphite electrodes, where lithium is inserted into the interlayers between the graphene sheets, silicon reacts with lithium and forms Li x Si alloys.4-6 Because the (de-)alloying reaction allows a higher lithium uptake per silicon atom (3579 mAh g −1Si , Li 15 Si 4 ) compared to the intercalation of lithium into the graphite host structure (372 mAh g −1 C , LiC 6 ), silicon offers an about ∼10 times larger theoretical specific capacity. However, while the intercalation chemistry reveals excellent cycling stability with only minor irreversible changes of the graphite's morphology (ca. +10%), 8 the (de-)alloying reaction causes significant morphological and chemical changes to the silicon particles, including (i) a large volume expansion of up to +280% and (ii) repeated breakage and formation of Si-Si bonds, which leads to severe mechanical stress and particle fracturing. [9][10][11][12] Upon continued cycling, these morphological changes cause a rapid capacity decay of silicon-based electrodes, which is largely driven by the electrical isolation of the fractured silicon particles.13-17 Nanometer-sized structures, including nanoparticles and nanowires, were shown to mitigate the mechanical stress which results from volumetric changes during the (de-)alloying reaction.12,18-20 However, there exists a trade-off, because the reduction of the particle size als...

Section: Resultsmentioning

confidence: 92%

Morphological Changes of Silicon Nanoparticles and the Influence of Cutoff Potentials in Silicon-Graphite Electrodes

Wetjen

Solchenbach

Pritzl

et al. 2018

102

135

“…52 While the lithium oxalate containing LNMO/graphite cells do have a ∼5-10% lower energy density, they are an interesting option for cobalt-free lithium ion battery cells, which may become critical in the future due to the rising cost and geographic concentration of cobalt. 53 Assuming all electrodes used here have the same initial porosity of 35%, the specific volume of electrode (including voids) per gram LNMO increases from 0.44 cm 3 /g LNMO to 0.46 cm 3 /g LNMO or 0.48 cm 3 /g LNMO by adding 2.5 wt% or 5 wt% lithium oxalate, respectively (calculated from electrode compositions given in Table I and bulk densities of 4.4 g/cm 3 for LNMO, 1.8 g/cm 3 for PVDF, 2.2 g/cm 3 for C65 and 2.1 g/cm 3 for lithium oxalate). 55 Accordingly, the oxidation of lithium oxalate leads to a porosity increase from 35% to 38% in the electrodes with 2.5 wt% and to 40% in the electrodes with 5 wt% lithium oxalate.…”

Section: Discussionmentioning

confidence: 99%

“…n CO 2 (el) n CO 2 (total) = V el RTc el V el RTc el + V gas K H [3] where n CO2(total) is the total amount of CO 2 present in the system and V gas is the volume of the cell's gas headspace. Assuming a constant gas volume, as would be the case for a hard-case cell, the pressure buildup can be expressed as 4: p = n CO 2 (total) K H RT V el RTc el + V gas K H [4] On the other hand, in soft pouch cells, gas evolution would typically lead to expansion (or bulging) of the cell.…”

Section: Discussionmentioning

confidence: 99%

“…For many layered oxide cathodes, however, the first cycle irreversible capacity of the cathode is similar to the capacity needed for SEI formation (∼20 mAh/g NMC ), and hence the practical discharge capacity of the cathode and the remaining active lithium are more or less balanced again. 2,3 In contrast, the first cycle irreversible capacity of LNMO (∼6 mAh/g LNMO ) is much lower than the capacity needed for SEI formation. This leads to a mismatch between active lithium and practical cathode capacity, i.e., there is not enough active lithium available to fully discharge the LNMO cathode during subsequent cycling.…”

mentioning

confidence: 98%

See 1 more Smart Citation

Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Solchenbach

Wetjen

Pritzl

et al. 2018

In the present study, we explore the use of lithium oxalate as a "sacrificial salt" in combination with lithium nickel manganese spinel (LNMO) cathodes. By online electrochemical mass spectrometry (OEMS), we demonstrate that the oxidation of lithium oxalate to CO 2 (corresponding to 525 mAh/g) occurs quantitatively around 4.7 V vs. Li/Li + . LNMO/graphite cells containing 2.5 or 5 wt% lithium oxalate show an up to ∼11% higher initial discharge capacity and less capacity fade over 300 cycles (12% and 8% vs. 19%) compared to cells without lithium oxalate. In LNMO/SiG full-cells with an FEC-containing electrolyte solution, lithium oxalate leads to a better capacity retention (45% vs 20% after 250 cycles) and a higher coulombic efficiency throughout cycling (∼1%) compared to cells without lithium oxalate. When CO 2 from lithium oxalate oxidation is removed after formation, a similar capacity fading as in LNMO/SiG cells without lithium oxalate is observed. Hence, we attribute the improved cycling performance to the presence of CO 2 in the cells. Further analysis (e.g., FEC consumption by 19 F-NMR) indicate that CO 2 is an effective SEI-forming additive for SiG anodes, and that a combination of FEC and CO 2 has a synergistic effect on the lifetime of full-cells with SiG anodes. Lithium nickel manganese spinel (LiNi 0.5 Mn 1.5 O 4 , LNMO) is a promising cathode material for high energy lithium ion batteries due to its high operating potential around 4.7 V vs. Li/Li + , its high rate capability, structural stability and the absence of cobalt. However, its lower specific capacity (146 mAh/g LNMO ) compared to layered oxide materials (e.g. lithium nickel manganese cobalt oxide (NMC), specific capacity 150-250 mAh/g NMC ) 1 is regarded as a major drawback. In full-cells, the practically achievable capacity of LNMO is even lower, as the formation of the solid-electrolyte interphase (SEI) on the graphite anode consumes active lithium. For many layered oxide cathodes, however, the first cycle irreversible capacity of the cathode is similar to the capacity needed for SEI formation (∼20 mAh/g NMC ), and hence the practical discharge capacity of the cathode and the remaining active lithium are more or less balanced again. 2,3 In contrast, the first cycle irreversible capacity of LNMO (∼6 mAh/g LNMO ) is much lower than the capacity needed for SEI formation. This leads to a mismatch between active lithium and practical cathode capacity, i.e., there is not enough active lithium available to fully discharge the LNMO cathode during subsequent cycling.In cells with silicon-based anodes, active lithium losses on the anode are even higher compared to graphite, as the expansion of the silicon particles during the first lithiation leads to a continuous exposure of fresh, unpassivated silicon surface. 4 On this new surface, electrolyte reduction occurs instantaneously, which reduces the total lithium reservoir in the cell. Therefore, different ideas to increase the amount of active lithium in lithium ion full-cells have been suggested, ...

“…We were hoping to improve the reversibility of the silicon composite by balancing the active material of cathode and anode, but obviously, further work is required to optimize the silicon composite anode to work in a full-cell, Li-ion battery as it was pointed out by other researchers. [44][45][46] Moreover, due to the challenges faced with this new type of full cells, it was hard to move to compositions with higher silicon content than 10 wt% until the issues with capacity fade are resolved. This, of course, requires the discovery of new electrolyte additives, binders and other components in the battery.…”

mentioning

confidence: 99%

Towards Improving the Practical Energy Density of Li-Ion Batteries: Optimization and Evaluation of Silicon:Graphite Composites in Full Cells

Yim

Niketic

Salem

et al. 2016

Increasing the energy density of Li-ion batteries is very crucial for the success of electric vehicles, grid-scale energy storage, and nextgeneration consumer electronics. One popular approach is to incrementally increase the capacity of the graphite anode by integrating silicon into composites with capacities between 500 and 1000 mAh/g as a transient and practical alternative to the more-challenging, silicon-only anodes. In this work, we have calculated the percentage of improvement in the capacity of silicon:graphite composites and their impact on energy density of Li-ion full cell. We have used the Design of Experiment method to optimize composites using data from half cells, and it is found that 16% improvements in practical energy density of Li-ion full cells can be achieved using 15 to 25 wt% of silicon. However, full-cell assembly and testing of these composites using LiNi 0.5 Mn 0.5 Co 0.5 O 2 cathode have proven to be challenging and composites with no more than 10 wt% silicon were tested giving 63% capacity retention of 95 mAh/g at only 50 cycles. The work demonstrates that introducing even the smallest amount of silicon into graphite anodes is still a challenge and to overcome that improvements to the different components of the Li-ion battery are required.