Wang Chunsheng \”natural energy\” rich electrolyte LiF new design of high performance booster micron alloy anode batteries
Because of its high theoretical capacity, a lithium alloy as an anode LixSi, LiyAl LizBi and the like, is considered to be one of the high specific capacity of the anode material for lithium-ion batteries (LIBs) most potential, attracts much attention. In many alloy material, micron Si, Al and Bi particles (SiMPs, AlMPs or BiMPs) because of its ease of manufacture, low production cost, low environmental impact and high compaction density, etc., the best anode material LIBs select. Although these materials exhibit a very high battery capacity, but also faced with the rapid decay of capacity, performance and low lethal circulatory problems and poor coulombic efficiency. This is mainly due SiMPs, AlMPs BiMPs and will have a greater volume expansion (up to 280%) to the alloy and the alloying process, the active material leads to the collapse of the fracture, thus losing the electrochemically active. Meanwhile, a huge volume of high activity so that shrinkage in the electrolytic solution surface is exposed, resulting in a solid electrolyte interphase (SEI) growing, and continue to consume the electrolytic solution, so that the coulombic efficiency of the cell (CE) is low, the difference between life cycle. Further, since the conventional organic electrolyte carbonates (e.g., FEC) formed – inorganic SEI film is not sufficient to accommodate the volume expansion cycle, SiMPs, AlMPs BiMPs anode material, and only the loop coil 20 will lose more than 40% of the capacity . Among the improved process, the electrolyte is considered to be one of the modified effective strategy, but so far none of the electrolytic solution can be modified LIBs micron average coulombic efficiency greater than 99.9% alloy anode.
[introduction] achievement
To address these challenges, the University of Maryland Professor Wang Chunsheng and the Army Research Laboratory Dr. Oleg Borodin [ 123] et al. a 2 M LiPF6 was dissolved in tetrahydrofuran (THF) and 2-methyltetrahydrofuran volume ratio of 1: 1 mixed solvent, the rational design of a new type of electrolyte LIBs (of LiPF6 -mix THF), so that SiMPs, AlMPs and BiMPs anode and commercialization LiFePO4 (LFP) or LiNi0.8Co0.15Al0.05O2 (NCA) of the positive electrode assembled full cell cycleMore than 100 times the life of the ring. Further, an area larger than 2.5 mAh cm-2 alloy anode of the first efficiency can be greater than 90%, and the average CE 99.9% greater than the cycle performance and stability of more than 300 turns. such excellent properties primarily due to the new LiPF6-mix THF electrolyte is formed at the interface of the high modulus organic thin LiF- interface, can improve the performance of the electrode material, and wherein LiF alloy anode having a high interface able to accommodate a lithium alloy plastic deformation during the cycle of. This work provides a simple, practical and efficient solution to the current battery technology, without modification or any special battery manufacturing method of adhesive. Relevant research results to \”Electrolyte design for LiF-rich solid-electrolyte interfaces to enable high-performance microsized alloy anodes for batteries\” in the title published in leading journals in the field of energy Nature Energy on, Chen Ji, Fanxiu Lin and Li Qin as a co-first author of the paper, Professor Wang Chunsheng and Dr. Oleg Borodin as a co-corresponding author. 
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First, the design principles alloy anode electrolyte
Currently the most commonly used electrolyte additive FEC will form an organic – inorganic SEI film, tightly affixed bonding a surface of the alloy, which makes the SEI film has a high alloy having the same amount of deformation, resulting in breakage and re SEI film is formed, and crushed and shedding of the active material (Figure 1a). Thus, the design of the electrolytic solution in targeting: forming a strong and having a low adhesion (high interfacial energy Eint) between the surface of the alloy of the SEI film, so that the stable material alloy at the interface to accommodate changes in volume (FIG. 1b). First, the researchers considered the case of silicon negative. Wherein, LiF because of the lithium silicate (Li4SiO4, fully lithiated oxide surface) and the surface has LixSiA higher Eint (FIG. 1c), SEI become suitable candidates. When the high modulus of LiF SEI film containing silicon is formed on the anode, and Li4SiO4 LixSi deformed and expanded, and little damage to the shell SEI. Meanwhile, the insulating properties of a wide band gap, and reduces the thickness of the SEI LiF, thereby increasing the initial CE. Furthermore, high modulus film may be effectively suppressed LixSi LiF pulverization. Although the use of an electrolyte containing FEC LiF can produce, but however, in addition to reducing the FEC also generate LiF organic components, thereby increasing the adhesion to the SEI LixSi, resulting in the expansion process SEI LixSi deformation and rupture. In view of this, researchers chose LiPF6 as a lithium salt, because it is reduced to produce organic by-products without LiF on the anode surface. LiPF6 were mixed with the electrolyte can be reduced at low potential, and the limited ability of Li salt solvent, so that the whole process of the lithium, LiF SEI film may be formed of preferentially LiPF6 high potential. LiPF6 and reduction potential depends on the degree of aggregation ions, an electrolyte having a high degree of aggregation to increase the reduction potential of LiPF6. Thus, the target should have a height of LiPF6 electrolyte salt aggregation, and the reduction potential of the solvent should be as low as possible to produce a rich LiF SEI film having a high purity. Ether having a lower thermodynamic reduction potential (0.0-0.3 V), which makes it sufficiently supports priority exploded fluoride (FIG. 1e). Wherein the salt increases the degree of aggregation (Fig. 1d) from the linear ether to mixTHF. In particular, LiPF6-mixTHF LiPF6 electrolyte high degree of association (FIG. 1d) such that the initial reduction potential higher than that of LiPF6 1.1 V, much higher than the reduction potential of mixTHF (FIG. 1e). Accordingly, expected to form a uniform LiF SEI layer in the above 0.1 V during lithiation, and Si lithiated end forms only a small amount of organic components on the LiF near the surface, so that the conventional SEI hybrid organic – inorganic composition stark Compared. This uniform organic thin LiF- the SEI, after the entire alloy lithiated form, and is expected to hold together the thinned lithiated alloy. Due to the high Eint LiF- alloying at the interface, in the lithium alloy of LiF elastic and plastic deformation may occur, thereby retaining the expansion alloy particles / shrinking process finishedIntegrity (FIG. 1b). Thus, the organic SEI LIF- double role played sturdy housing, the flow will break or an alloy held firmly together (Figure 1a).
Figure 1. Effect of SEI and electrolyte performance alloy anode particles. After the combined organic (a, b) circulating, low Eint, and inorganic non-uniform, high Eint, uniform principle lithium alloy interface -SEI comparison chart; (c) Li alloy -LiF local electronic interface functions and Eint; (d ) MD simulation LiPF6-mix THF (2 M) and LiPF6-EC-DMC (1 M) of an electrolyte distribution Li + solvent; (e) QC Li + calculated first restore critical electrolyte components solvated shell potential.Second, the electrochemical performance of the anode half cell SiMPs
using an electrolyte LiPF6-mix THF SiMPs assembled half cell cycle performance: the area of the active material loading of 2.0 mg cm-2 when , Si anode showed 5.6 mAh cm-2 and 2800 mAh g-1 at a high capacity rate C / 5, and the post-cycle capacity retention ring 400 was 90%. Meanwhile, the first high coulombic efficiency of 90.6%, and the second lap CE greater than 99.9%. In sharp contrast, in 1 M LiPF6-EC-DMC electrolyte, Si anode circulation after 20 laps will lose about 40% of capacity, after 50 laps, leaving only about 8% of capacity, and kept at the CE 96 to 97%. Performance rate: the rate at 1C, a silicon anode can stay above 2400 mAh g-1 high capacity, even at high magnification. 3C, still exhibits high as 1580 mAh g-1 capacity. Meanwhile, the use of LiPF6-mix THF SiMPs assembled half cell electrolyte solution at low temperature exhibits excellent properties. When the temperature dropped to -20 and -40 ℃, still has a silicon electrode 2304 and 1475 mAh g-1 reversible capacity. Traditional electrolyte, only a 658 and 0 mAh g-1 capacity.
Figure 2. The electrochemical performance of the anode half cell SiMPsthree, AlMP and BiMP electrochemical performance of the anode half cell
AlMP anode: at magnification of 2C, the circular ring 260 No significant capacity fade was observed, and the first effect of 91.6%, beginning from the eighth circle CE was higher than 99.9%. Further, even at high rates of 30C, still more than 50% of the initial capacity. When the current density returned to 2C, to restore the discharge capacity of about 900 mAh g-1 capacity, which proves an anode resistance AlMP rapid phase transition. BiMP anode: at magnification of 2C, ring 250 cycles was not observed significant capacity fade, greater than 99.9% and the cycle CE. 60C while at high rates, and can keep the same 50% of the initial capacity.
Figure 3. The electrochemical performance of the anode half cell AlMPIV, SEI film chemical analysis
the XPS analysis showed that the top of an organic SEI (RCH2OLi ) and inorganic (Li2O, LiF) two components, and the carbon content (expressed organic decomposition products) decreases with increasing sputter time. Specifically, in the spectrum Si, Li4SiO4, Si, and Li-Si alloy – signal dominant, and during sputtering 600 s, Li-Si alloy signal reaches 50% of all signals Si is considered SEI-Si interface. C1s signal falls before reaching the SEI-Si interface to the noise level, accompanied O1s spectra related to carbon reduction O = C = O signals. Meanwhile, LiF signal is still very strong in SEI-Si interface, and persist throughout the sputtering process, uniform organic thin SEI LiF- this proposed structure of Figure 1b. Further, a peak was not observed in the new SiOx LiPF6-mix THF electrolyte circulation silicon, further confirmed due to the formation of a uniform SEI, SiOx layer having a uniform and complete lithiation.
FIG. 4. Si are circulated in the anode 2 M LiPF6-mix THF (a) and 1 M LiPF6-EC-DMC (b) electrolyte.Fifth, an anode circulation morphology characterization Si
Researchers situ electrochemical atomicForce microscopy (AFM) to study the dynamic lithiation / delithiation roughness and thickness of the SEI, ultra-smooth and selecting a silicon wafer (roughness of about 0.18 nm) was observed object. In 2 M LiPF6-mix THF electrolytic solution, the silicon wafer is increased roughness lithiated state ~ 1.78 nm, the roughness is reduced to delithiated ~ 1.01 nm (FIG. 6A), than a conventional electrolytic solution and 3.87 4.06 nm ( FIG. 6b) is much smaller. Furthermore, consistent with the different degrees of roughness XPS Si2p spectra, indicating that the SiOx 2 M LiPF6-mix THF electrolyte uniformly fully lithiated. Furthermore, the design of the electrolytic solution in the lithiation process to reduce the roughness reflects LiF- organic thin SEI suppressed irregular volume expansion and Si are held together, which is a conventional electrolyte solution mixed organic – inorganic SEI can not be achieved (after delithiation roughness increases). Further, SEM confirmed the high Eint between Si and Li4SiO4 after LiF SEI with lithiated, beneficial adaptation lithiation expansion and contraction of Si, thereby forming the core 2 M LiPF6-mix THF electrolyte – shell structure , and formed in a conventional organic electrolyte solution – the so LixSi SEI inorganic particles or Si powder.
Figure 5. Morphological characterization of silicon after the anodic cycleSixth, the full cell characterization
Finally, the researchers were constructed SiMP-, AlMP- and BiMP // LFP (loading area of 2.3 mAh cm-2) and SiMP-, AlMP- and BiMP // NCA (loading area of 1.6 mAh cm-2) and tested for full cell performance. In the absence of any pre-lithiation procedure, all of the full cell and the actual current density in the capacity test area can exhibits stable cycle characteristics and high close to 100% CE (after cycling five laps). Moreover, no significant voltage changes observed during the cycle, indicating that electrodes and electrode – electrolyte interface is very stable during cycling. Further, 2 M LiPF6-mix THF electrolyte, SiMP // NCA full cell after 30 cycles, retained about 92% of the initial capacity, SiMP // LFP full cellAfter 100 cycles, the capacity retention rate of 80%, much higher than the added FEC additive containing a conventional electrolyte solution of 4.5% and 6.2%.
FIG 6. SiMP-, AlMP- BiMP // LFP and cycle performance of the full cell[Conclusion] Outlook
