Promoting Si-graphite composite anodes with SWCNT additives for half and NCM811 full lithium ion batteries and assessment criteria from an industrial perspective

Abstract

Single wall carbon nanotube (SWCNT) additives were formulated into µm-Si-graphite composite electrodes and tested in both half cells and full cells with high nickel cathodes. The critical role of small amount of SWCNT addition (0.2 wt%) was found for significantly improving delithiation capacity, first cycle coulombic efficiency (FCE), and capacity retention. Particularly, Si (10 wt%)-graphite electrode exhibits 560 mAh/g delithiation capacity and 92% FCE at 0.2 C during the first charge-discharge cycle, and 91% capacity retention after 50 cycles (0.5 C) in a half cell. Scanning electron microscope (SEM) was used to illustrate the electrode morphology, compositions and promoting function of the SWCNT additives. In addition, full cells assembled with high nickel-NCM811 cathodes and µm-Si-graphite composite anodes were evaluated for the consistence between half and full cell performance, and the consideration for potential commercial application. Finally, criteria to assess Si-containing anodes are proposed and discussed from an industrial perspective.

Keywords

lithium-ion battery / Si anode / Si-graphite composite / single wall carbon nanotube (SWCNT) / NCM811

Introduction

Lithium-ion batteries (LIBs) have become the mainstream choices for the emerging sectors of portable electronics, energy storage, electric vehicles (EV), smart grids, and robotics etc. due to their high gravimetric and volumetric energy density [¹,²].The conventional rechargeable LIBs mainly consist of four parts: cathode and anode electrodes, separator and electrolyte. The cathode is the component carrying the Li ions during the process of electrode preparation and battery assembly; while the role of the anode is to receive/release Li ions from/back to the cathode during the charge/discharge cycle for LIBs to store/provide the electric powder. The two electrode components are the active parts deciding the theoretical energy density of a battery. Pursuing the upper limit capacity of these conventional electrode materials has reached the bottleneck in recent years. Motivated by the hundreds of billion dollars LIB market in the next 20–30 years, searching even higher capacity cathode and anode materials has attracted intensive attention in the past decade [³–⁸].

In the case of anode, graphite is dominantly used in current commercial LIBs because of its high electronic conductivity, high first cycle coulombic efficiency (FCE, 90%–95%) and cycling CE (CCE,>99.9%), and relatively low cost. However, graphite has a theoretical capacity of only 372 mAh/g since in graphite, every six carbon atoms can hold only one Li ion. Since the first commercial LIB developed by Sony Corporation in 1991, R&D and capacity improvement using graphite-based anode have been pushed to the upper limit, as close as possible to the theoretical value. With the increasing needs of high energy density battery pack, alternative anode materials with higher theoretical capacity, e.g., Si, SiO, Sn, and Li metal etc., have been intensively studied [⁹–¹²]. Among these options, Si is considered the closest option for commercialization to replace graphite. Since each silicon atom can bind up to four Li ions, this gives more than ten times capacity (4212 mAh/g for Li_4.4Si at high temperature and 3579 mAh/g for Li₁₅Si₄ at room temperature) than graphite anode [¹³,¹⁴]. In addition, Si is the second abundant element in the earth crust, environmentally benignity, low electrochemical potential (~ 0.5 V vs. Li) and similar bulky density to that of graphite (~ 2.3 vs. 2.2 g/cm³). However, Si has a volume change of ~ 300% when receiving Li ions during battery cycling, which leads to severe particle pulverization and unstable solid-electrolyte interphase (SEI) formation, and causes losses of electrical contact in the electrode level. Moreover, Si is a semiconductor in essence. A low electronic and ionic conductivity will also limit its electrochemical performance (e.g., capacity utilization rate and rate performance) in a LIB. In the end, a LIB using Si anode usually shows low FCE and CCE, fast capacity fading and limited cycle life. To address these issues (mainly volume expansion and conductivity), significant efforts, e.g., nano-Si [⁷,¹⁵], nanowires [¹²], surface carbon coating [¹⁶,¹⁷], york-shell structure [¹⁸], porous Si [¹⁹,²⁰], Si alloys [²¹], Si hybrids [²²], Si-carbon (including graphite, graphene and carbon nanotubes etc.) composites [²³–²⁹] and pre-lithiation [³⁰,³¹] etc. have been extensively pursued. Some review papers of the above work and other endeavors can be found in the references therein [⁶–⁸,³²]. Although there have been many breakthroughs claimed on these Si-containing anodes, most of the above efforts still stay at the academic R&D level. For industry application in a short-term, the Si-carbon composites prepared by doping small amounts (3 wt%–20 wt%) of Si or modified Si powders into graphite are considered to be more realistic to boost the anode capacity. Nevertheless, R&D progress achieved in both academia and industry in the past decade let people believe that Si or Si-containing anodes are to be widely commercialized within the next 5–10 years.

At the academic R&D level, a coin half-cell is frequently assembled to test and screen new Si-containing anode materials. In general, four groups of data need to be collected for evaluation: ① first cycle delithiation capacity at a slow C-rate, ② FCE and CCE, ③ capacity retention and/or rate capability, and ④ electrode physical properties including active mass wt%, total mass loading (mg/cm²) and press density (mg/cm³). The other information such cost, safety and large full cell performance etc. that are the targets for the future commercialized manufacturing process is usually not addressed at the R&D stage. Normally, most of the academic reports emphasize achievements from two or three groups where asunder state others. A typical example is high capacity (e.g., close or>1000 mAh/g) and capacity retention (>90% after 100 cycles or longer) were achieved on new powders, but ended in low FCE (<70%) and CCE. Moreover, the as-achieved powders were generally not tested in full cells, which is useful to check the applicability of the half-cell claims and potentially leads to scalable products. Overall, the assessment criteria of the performance of Si-containing anodes are needed to narrow the gap between the academic R&D and industrial applications.

In this report, we investigated the critical role of single wall carbon nanotube (SWCNT) additives in µm-Si-graphite composite anode in both half and full LIB coin cells. Since high nickel NCM811 and NCA cathodes and Si-graphite composite anode are considered to be the next generation electrode dual to achieve high energy density batteries for electric vehicle (EV) [³³], it is of our interest to study the full cells built with NCM811 and Si-graphite composite for the first time to evaluate these promising electrode materials for potential industry application. Afterwards, focusing on the four groups of key R&D data from an industrial perspective, we propose and discuss the assessment criteria on how to evaluate and transform high-performance Si-containing anodes identified from academic R&D into commercial products.

Experimental

Materials and characterization

Si powder used in this work was purchased from Xuzhou, China. Single-wall-carbon nanotube (SWCNT) in 2 wt% solution was purchased from OCSiAl LLC. Graphite powder was purchased from Zichen, China. Scanning electron microscope (SEM) images and EDS of Si powder and electrodes were collected using a SEM (Verios 460, FEI) with an energy dispersive X-ray spectroscopy (EDXS) system (Oxford Instruments, with Aztec analysis software) coupled to the electron microscope. An X-ray photoelectron spectrometer (K-Alpha, ThermoFisher) was used to determine the chemical composition of the surface of the materials. The uncertainty of the element composition was conservatively estimated as ±3%. The crystallinity of the annealed particles was probed using X-ray diffraction (XRD) (D8 Advance, Bruker) using a CuKa source. Backgrounds were fitted though the raw data and subtracted.

Coin cell fabrication and performance test

All Si-graphite composite anode electrodes (except Si-only anode) were prepared in the composition of weight (wt)%: 95.5% active material, 1% carbon black SuperP (SP) and 3.5% binders (SBR/CMC= 2:1). When 0.2 wt% SWCNT was added, the weight was deducted from SP. The components were mixed with H₂O in a planetary milling jarstirred for 30 min. The resulting slurry was cast onto copper foil and dried in a vacuum oven. Si-only anode electrode in this work was prepared in weight percentage of 70% Si powder, 20% SP and 10% binder (SBR/CMC= 2:1). Half-cells were assembled using a Li counter electrode in a 2032 coin cell configuration with an electrolyte solution consisting of 1.2 M LiPF₆ in a 1:1:1 ethylene/dimethyl/diethyl carbonate solution plus additional 10% fluoroethylene carbonate. For coin full cells, NCM811 cathode electrode was prepared from 90% active material, 4% SP and 6% PVDF binder, mixed with N-methyl-2-pyrrolidone solvent in a planetary jar and stirred for 30 min. The resulting slurry was cast onto aluminum foil and dried in a vacuum oven. For anode electrode, we kept mass loading of 5–6 mg/cm² and press density of 1.2–1.3 mg/cm³ after calendaring the electrode.

To determine the lithiation/delithiation (corresponding to charge/discharge in a full cell) characteristics of the anode materials, the half-cells were cycled between voltages of 1.5 to 0.02 V at rates of 0.2 C in the first cycle, then 0.5 C from the second cycle. For full NCM811 coin cells, the cells were cycled between the cutoff voltages of 2.8 and 4.25 V at the rates of 0.2 C (first cycle) and 0.7 C/1 C (from 2nd cycle).

Results and discussion

Silicon powders

The XRD/SEM/EDS characterization results of the Si powders are shown in Fig. 1.The as-received Si powder shows well-defined peaks indexed to (111) at 28.24°, (220) at 47.12°, (311) at 55.96°, and (400) at 68.96°. SEM image shows the powder is ~ 2 µm in length (Fig. 1(b)). µm-Si powders other than the nano-size chosen in this study were mainly for processing and electrode design consideration. Although the nm-Si powder, especially the size under 150 nm, was reported being able to address the particle cracking problem due to volume expansion [³⁴], the use of nm-Si powders in a large amount is not just difficult to handle due to their small size and lightness, direct processing under the industrial environment will result in severe surface oxidation forming an insulating SiO₂ layer, which reduces both the specific capacity and electrochemical activity. The µm-Si powders used in this work have less than 6 wt% oxygen (Fig. 1(c)) and are stable in the atmosphere. On the other hand, when ~2 µm-Si powders are mixed with graphite particles which are usually in the size range of ~20 µm, the overall powder tap density has less change due to the pores existing inter-graphite particles. As such, half coin cells were built and tested to collect electrochemical performance data for the electrodes made from powders: (a) Si-only, (b) graphite reference, and (c) Si-graphite composite, and the results are shown in Fig. 2.

Battery performance of Si-only, graphite and Si-graphite composite electrodes

Figures 2(a)–2(c) show lithiation/delithiation voltage (V) profiles for Si-only, graphite and Si (3wt%)-graphite composite electrodes in half-cells, respectively. Three electrodes present first cycle delithiation capacity (0.2 C) and FCE of ~2550 mAh/g and 73%, ~ 344 mAh/g and 95%, and 393 mAh/g and 89% (summarized in Table 1), respectively. The V-plateaus of the first delithiation curves for Si-only and graphite electrodes are ~ 0.44V and ~ 0.2V (Figs. 2(a) and 2(b)), respectively. For Si-graphite composite electrode (Fig. 2(c)), there is a clear V-plateau transition from lithiated graphite to lithiated Si starting ~0.3 V, which is further confirmed by the first delithiation V-plateau comparison of the three electrodes (Fig. 2(d)). A flat V-plateau in the first delithiation cycle provides useful information on Si powder properties, e.g., crystallinity, size distribution, electrode composition and potential electrochemical performance. Broad slope changes in V-profile without flat plateau usually indicate morphology-dependent performance or unwanted delithiation process that are not suitable for most electronics and EV batteries requiring a stable V source [³⁵].

As shown in the V-profiles evolving trend vs cycles in Fig. 2(a), a significant V-shifting (V-plateau lost) and capacity fading was observed on the Si-only electrode, while there was no noticeable fading on graphite electrode (Fig. 2(b)). As for the Si (3 wt%)-graphite composite electrode, V-plateau due to Si was partially lost after 20 cycles (Fig. 2(c)). Capacity retention vs. cycles was compared in Fig. 2(e). Si-only electrode loses more than 90% capacity in only 5 cycles, while graphite electrode has 0.5% loss in 20 cycles (retention was calculated from 2nd cycle, so was the composite electrode), and the Si (3 wt%)-graphite composite electrode lost about10% total capacity. This 10% loss in the capacity can be ascribed to the degradation of the Si powders in the anode composite (about half of Si became inactive judging from the change in V-plateau) because the graphite electrode has negligible capacity drop (Fig. 2(b)). In this work, the Si-graphite composite electrode has much less Si (3 wt%) than Si-only electrode (70%), therefore, there are more electrical contacts between Si and graphite particles in the composite and improved capacity retention for Si particles. Nevertheless, the 10% capacity loss in 20 cycles is still too large for this composite powder to be used as the anode in a commercial full cell. Thus, we explored to improve the electrochemical performance of the Si-graphite composite by blending small amount of Single Wall Carbon Nanotube (SWCNT, 2–3 nm-size in width and 4–5 µm in length), as the additional conducting additive.

Effect of the SWCNT additive on improving Si-graphite composite performance

Carbon nanomaterials such as carbon nanotubes (CNT), including SWCNT and grapheme are characterized by strong physical strength and extremely high electrical conductivity. Those materials have been extensively investigated as either the direct anode materials or the hosts for Si to improve the electrochemical performance of Si-containing anode [²⁵,²⁷,²⁸,³⁵–³⁷]. The direct use of carbon nanomaterials as the anode is not successful due to the high irreversible lithium ion capacity loss inherited on those carbon materials [³⁴]. When being used as the hosts, improved electrode capacity and capacity retention were reported at very high Si wt% loadings (10 wt%–80 wt%), however, an unsatisfied low FCE ~ 70%±10% was usually obtained. Thus, in our study, we investigated SWCNT as the conducting additive with the objective to systematically improve the electrode electrochemical performance. Although SWCNT as the additive for Si anode has been reported in industry [³⁸], to the best of our knowledge, there are no literature reports on using low concentration of SWCNT additives (<1 wt%) directly in the electrode level for Si-anode. In this work, we investigated the effect of SWCNT as the additive in the Si-graphite composite electrode and conducted systematic half and full cell studies. The results are summarized in Table 1 and shown in Figs. 3(a)–3(b).

Capacity, FCE, retention and rate capability

The electrochemical performance can be significantly improved by SWCNT addition. For example, the first delithiation capacity of the Si (3 wt%)-graphite anode improved from 393 to 406 mAh/g, and the FCE improved from 89.3% to 92.9% after adding 0.2 wt% SWCNT (Table 1). In Fig. 3(a), the voltage plateau (width) related to the delithiation of Si almost has no observable change from cycle 2 to 20, and there is only 3% capacity loss (Fig. 3(b)) (vs. 10% loss for the electrode without adding SWCNT shown in Fig. 2(c)). Larger improvement by SWCNT addition was achieved for the electrode of Si (5 wt%)-graphite composite with the capacity improved from 375.8 to 455 mAh/g, and FCE improved from 81.2% to 92.9%. Besides these, SWCNT additive can also improve rate performance significantly. The delithiation capacities of the electrodes with and without SWCNT at different C-rates (0.1, 0.2, 0.5, 1 and 2 C) were measured and compared in Fig. 3(c). At 2 C, the rate capability (normalized over 0.1 C) for the electrode with SWCNT addition is 10% higher than that has no SWCNT (93% vs. 83%). It is worth pointing out that we have formulated electrodes with 0.1 and 0.4 wt% SWCNT additive, and studied the above performances in half cells. The results were found similar to those of 0.2 wt% SWCNT. Therefore, we did not present data at here; while the lower limit of SWCNT amount deserves future exploration.

To examine more of SWCNT effect on the electrode performance, we studied SEM images of the electrodes with and w/o adding SWCNT shown in Fig. 4.

Surface morphologies of electrodes containing SWCNT additives characterized by SEM

SEM images of the electrodes with SWCNT additives are in Figs. 4(a)–4(c). As a comparison, the SEM image of the electrode without SWCNT additives is shown in Fig. 4(d). Graphite particles, Si particles, conducting carbon black SuperP (SP) additives and SWCNT additives are marked as shown in Figs. 4(b) –4(d). Although the nano-size, round-shape SP particles were formulated as the electronic additives, they aggregated into large and separated particles which cannot function as the conducting channels for different Si particles (Figs. 4(c)–4(d)). Better physical contact can be obtained by SWCNT additives. As shown in Figs. 4(b)–4(c), the 4–5 µm long SWCNT form bridges to connect Si-Si and Si-graphite particles. These bridges are expected to increase the electrical conductivity of the anode composite, thus improve the electrochemical performance. After cells cycled over 50 cycles, we could still observe SWCNT bridges between Si and graphite particles (Fig. 4(e)). This observation is consistent with the comments that SWCNT can bend and wrap particles together [³⁵]. In addition, we systematically studied Si-graphite composite electrodes with the Si wt% varying from 1.5% to 20%, in which 0.2 wt% SWCNT additives are added. The half coin cell performance data are summarized in Table 1 and the corresponding characterization curves are shown in Figs. 5(a)–5(d).

Si (1.5 wt%-20 wt%)-graphite composite, capacity, FCE/CCE and electrode design

Table 1 lists first lithiation/delithiation capacity, FCE, and capacity contribution from Si calculated based on the graphite capacity of 344.1 mAh/g. It is interesting to note that the Si capacity contributions are close to 2500 mAh/g for all of the composite electrodes with SWCNT additive (except for the electrode containing 1.5 wt% composite electrode in which the Si contributes 2930 mAh/g). Without SWCNT, Si capacity contribution is only 1981 mAh/g and 980 mAh/g for 3 wt% and 5 wt% Si-graphite composite electrodes, respectively. Figures 5(a)–5(b) show the capacity and capacity retention vs. cycles for each composite electrode. At 0.5 C cycling, Si (3 wt% and 5 wt%)-graphite electrode show 97% retention after 50 cycles. The capacity retention dropped to 91% for the Si (10 wt%)-graphite electrode. As the Si increases to 20 wt%, the capacity dropped much faster than others.

As more Si is added into the composite anode, less active anode mass is needed to provide the same capacity as the graphite for full cell cycling, resulting in higher gravimetric energy density (Wh/kg). Figure 5(c) shows the first cycle delithiation capacities of the Si (1.5 wt%-20 wt%)-graphite composite electrodes and corresponding active anode mass that can be saved after replacing partial graphite with Si. However, more Si loading means larger volume expansion of the electrode at the same time. For commercial batteries using graphite anode, the total anode mass loadings usually reach 10–15 mg/cm² with press density of 1.4 g/cm³ or higher in order to reach the requirement of volumetric energy density (Wh/L) [³²]. The graphite electrode after being calendered has porosity around 35%, which is large enough to accommodate the volume expansion of graphite (<10% expansion in fully lithiated state), so the cell performance is very stable. For the Si-containing anode, the limited of porosity will significantly affect Si cycling (~300% volume expansion in lithiated state) and cell performance. A theoretical calculation shows that the Si wt% should be less than 11.68 wt% in Si-graphite composites [³⁹]. Beyond this limit, the electrode porosities cannot hold the overall volume expansion and result fast electrode failure. This should partially explain the observation of fast capacity drop for the Si (20 wt%)-graphite electrode in our work. Thus, the balance between gravimetric and volumetric energy density as well as the cell electrochemical performance needs to be considered altogether in the future when designing electrode composition and full cell. The electrode design and physical properties will be discussed further in the Section 3.3.5.

The half-cell data from the above Si (1.5 wt%–10 wt%)-graphite composites have shown promising results including capacity, FCE, and capacity retention. However, more investigation is still needed before claiming they are promising anode candidates for commercial full cells. The supply of Li is unlimited when Li metal is the anode in half-cells. In full cells, the source of Li is only provided from the pre-loaded lithium in the cathode materials. Different amounts of lithium in half-cells and full cells may affect the performance of the anode. Therefore, it would be necessary to compare the anode performance in half and full cells, and test whether the electrochemical performance such as CE/CCE and capacity retention in half-cell is consistent with the performance in full cells.

As shown in Fig. 5(c), the CCEs of all half-cell electrodes are compared. The CCE of the graphite reference is between 99.9%–100%, while, the CCEs of the 3, 5, 10, and 20 Si wt% composite electrodes are 99.7%–99.9%, 99.5%–99.7%, 99.2%–99.4%, and<99.0%, respectively. For industry applications, full cell performance usually requires 80% capacity retention over 500–1000 cycles, hence this requires the full cell CCEs to reach or be higher than 99.9% during the cyclings. To meet this requirement, the half-cell CCEs of the anode (and cathode) materials need to be as high as possible (99.9% is the minimum). To correlate the half-cell data with the full cell performance, especially CCE performance, we built full coin cells which are assembled with the cathode made of home-developed NCM811 [⁴⁰] and the anodes made of graphite (without SWCNT), or 3 wt% and 5 wt% Si-graphite containing SWCNT additives. The fuel cell cycling performance is shown in Fig. 6.

Full cell performance with NCM811 cathode

Figure 6(a) compares the first charge/discharge cycles of the three full cells of graphite (without SWCNT), or 3 wt% and 5 wt% Si-graphite containing SWCNT additives at 0.2 C. In discharge curves, there are obvious slope changes for the anodes containing Si, corresponding to the delithiation transition from graphite to Si in half-cells (Fig. 3(a)). Discharge voltage profiles also show the trend of the decreasing V-plateaus with the increasing Si wt%, which corresponds to the higher Si V-plateau vs. graphite (0.44 V vs. 0.2 V) in the half-cell performance. The full cell FCEs are 86.9%, 85.3%, and 84.1% for the anodes using graphite, 3 wt% and 5 wt% Si-graphite, respectively, about 1% difference (NCM811 cathode in this work has FCE of ~87% in half-cell [³⁹]). At 1 C cycling, the capacity retention after 50 cycles is 96.5%, 85% and 72% (Fig. 6(c)), respectively. As shown in the previous section in half cells, capacity retention for both 3 wt% and 5 wt% Si anodes are ~97%, while in full cells, there is more than 10% difference after 50 cycles. The reason can be explained from CCE values.

Full cell CCEs are 99.9%–99.99%, 99.6%–99.7% and 99.1%–99.2% for the cells using graphite, 3 wt% and 5 wt% Si anodes, respectively, and data are compared in Fig. 6(d). Although the difference in CCEs is less than<1% for each electrode, the small difference is accumulated into large gaps in retention among three full cells after extended cycles. As discussed earlier, the half-cell CCEs for graphite, 3 wt% and 5 wt% Si-graphite anodes are 99.9%–100%, 99.7%–99.9%, and 99.5%–99.7%, respectively, which are decreasing in the same trend as those of full cells, therefore, it is reasonable to deduce that the half-cell data have direct connection on full cell performance. If half-cell CCEs cannot reach>99.9%, it is hard for full cells to have high capacity retention.

It is worth noting that our coin full cells using NCM811 cathode and Si-graphite composite anode have gravimetric energy density of ~750 Wh/kg calculated based on total cathode mass, which has reached the requirement for high energy EV applications [³²]. Our next step is to build a multilayer pouch cell with more consistent cell thickness information to evaluate both the Si-graphite composite anode and cell’s volumetric energy (Wh/L) for the future industry applications.

Assessment criteria for Si-containing anodes

As mentioned earlier that in academic reports, there have been so many achievements claimed on the Si-containing anodes, but few has been successfully transferred from research to commercial products. To have a clear judgement on a new material or a novel process, and guide academic R&D activities from an industrial perspective, assessment criteria of the performance are therefore needed. On the basis of our own achievements and literature results, we propose three criteria on how to assess the electrochemical performance of a new Si-containing anode from the half-cell data: ① First cycle delithiation capacity should be considered together with FCE. The capacity achieved on Si-containing anodes largely depends on the Si weight percentage in an electrode. A higher capacity anode does not mean a better candidate for full cells. For example, if a low FCE is obtained, e.g., 60%–70%, there would be significant Li loss in the formation cycle, making it impractical for commercial cells. Since the FCEs of the commercial graphites are between 90%–95%, we propose the Si-containing anodes should have FCE close to that of graphite, e.g., 90% to be competitive without reducing the advantage of high capacity. ② Capacity retention performance should be analyzed together with CCE. As shown in our work on Si (3 and 5 wt%)-graphite composites, although both electrodes have very high and similar retention ~97%@50 cycles, CCE of Si (3 wt%)-graphite is 0.02% difference. After they were built into the full cells, capacity retention of Si (3 wt%)-graphite became 10% higher (85% vs. 72.1%) at 50 cycles. So, even an anode has very promising retention performance, if its CCE is low (<99.9%), the capacity retention will still drop fast in full cells. For industrial application, half-cell CCEs together with retention are important factors predicting full cell cycling. ③ Electrode composition and press density need to match those of commercial graphite anode. The electrode physical properties include active mass wt% (excluding binder and additive), total mass loading and press density. In academic studies, the anode active mass in an electrode is usually less than 90 wt% with total mass loading less than 3 mg/cm², see examples in some recent references [¹⁶,⁴¹–⁴³]. Concerning press density of an electrode and as mentioned in the Section 3.3.3, the commercial calendered graphite electrode can reach>1.4 g/cm³ with porosity ~ 35%. In academic R&D, the electrode generally has no calendering process or only a light calendering, which gives electrodes with press density normally less than 1 g/cm⁻³ and much higher porosity%. More porosity benefits Si powder cycling as discussed. So the press density is of critical importance for Si-containing anode and should be added in the assessment criteria too. We propose tomimic industry graphite formulation conditions, e.g., active mass wt% should be larger than 90% in an electrode composition; while mass loading might not be crucial (>5 g /cm²), but press density should be as close as possible to that of graphite anode. It is worth noting that our study on Si-graphite composite anode with SWCNT additives has been using the conditions closer to the industry standards, e.g., active material is 95.5 wt%, mass loading is controlled between 5 and 6 mg /cm², and electrode press density is ~1.3 g/cm³. Overall, the above assessment criteria emphasize a comprehensive consideration of a new Si-containing material instead of one or two shinning achievements from R&D results for potential industry application. If all the above assessment criteria can be met, it would then reach the step to evaluate cost and safety etc. parameters for industry process, which is beyond the scope of this report.

Conclusions

In summary, we have prepared µm-Si-graphite composite electrodes and studied the effects of SWCNT as the additive in both half and full LIB cells. The results show that the cell performance, e.g., the capacity, FCE/CCE, and capacity retention, can be significantly improved with 0.2 wt% SWCNT additives. The SEM images show that SWCNT can effectively build the new connection among the neighboring Si-Si and Si-graphite particles. Si wt% varying from 1.5 to 20% relative to the graphite composite was studied systematically in half-cells and full cells using NCM811 as the cathode materials. Although we found that Si-graphite composite with SWCNT improves the cell capacity, FCE, and capacity retention in a half cell with less than 10 wt% Si, the full cell performance, especially capacity retention, was still low. The low capacity retention is correlated to the low CCEs in both half and full cells. Aimed at narrowing the gap between academic R&D and industry applications, we proposed three assessment criteria to evaluate a new Si-containing anode: i) first cycle delithiation capacity should be considered together with FCE, ii) capacity retention performance should be analyzed together with CCE, and iii) electrode composition and press density need to match those of commercial graphite anode. We conclude that a new Si-containing anode material and the related process can be claimed to be promising for commercial products only all above criteria meet the industrial requirements.

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