Mechanochemical Synthesis and Electrochemical Properties of Li x VS y Positive Electrodes for All-Solid-State Batteries

To enhance the energy density of all-solid-state batteries, polysulfide positive electrodes have a great advantage of their high capacity. In this study, we developed Li x VS y (x = 5–9, y = 4–6) comprised Li2S and LiVS2. Although Li2S is an insulator, Li x VS y shows a high electronic conductivity (∼10−1–10−2 S cm−1) because it contains LiVS2 with a high electronic conductivity. The theoretical capacity of Li x VS y is 626–789 mAh g−1 when all the Li in Li x VS y reacts. Li x VS y positive electrodes achieve a high energy density because they show high capacity with no conductive additives and high loading of Li x VS y .

Sulfur-based active materials for positive electrode of high energy batteries have attracted global attention because of their high capacities and low costs. 1,2 However, lithium batteries with sulfur-based positive electrodes exhibit capacity fading because polysulfides are formed during charge-discharge reactions and dissolve in organic liquid electrolytes. 3,4 All-solid-state lithium batteries with sulfur-based positive electrodes can prevent the dissolution of polysulfides. 5,6 Sulfide-type all-solid-state lithium batteries have been developed as power sources for electric vehicles because sulfide SEs exhibit high ionic conductivities of ∼10 −2 -10 −3 S cm −1 at 25°C. [7][8][9] Compared with oxide-positive electrodes such as LiCoO 2 and LiNi 1/3 Mn 1/3 Co 1/3 O 2 , sulfur-based positive electrodes show higher formability, resulting in interfaces between sulfur-based positive electrodes and sulfide SEs that are expected to maintain sufficient contact during the charge-discharge processes. 10 When high-voltage oxide-positive electrodes are used for sulfide all-solid-state batteries, oxide buffer layers such as LiNbO 3 between the positive electrodes and sulfide SEs are required to prevent the decomposition of sulfide solid electrolytes and the formation of a high-resistance layer. 11 In contrast, sulfur-based positive electrodes require no buffer layers because their operating voltages do not greatly exceed the potential window of sulfide solid electrolytes. 12 Sulfur-based positive electrodes generally exhibit low electronic conductivity due to the low electronic conductivity of S and Li 2 S, which is a big issue. Therefore, many conductive additives are needed to use sulfur-based positive electrodes, resulting in low energy density due to the low amount of sulfur-based positive electrodes. The use of metal polysulfides is an effective approach to achieving high electronic conductivity. Rout et al. reported a VS 4 -based hybrid anode material by growing single-crystalline VS 4 nanostructures on the reduced graphene oxide (rGO). 13 Zhang et al. reported that nanocomposite of 10%rGO-VS 4 and Li 7 P 3 S 11 solid electrolytes showed better rate capability and cycling stability. 14 Crystalline VS 4 was reported as a semiconductor with a band gap of about 1.0 eV. 13 We have developed an amorphous VS 4 (a-VS 4 ) as an active material for a positive electrode by tuning its operation voltage and crystal structure. 15,16 The a-VS 4 exhibits a high electronic conductivity of 3.3 × 10 −5 S cm −1 , resulting in that lithium-ion batteries using the a-VS 4 operated with few conductive additives. 16 After the lithiation process of the a-VS 4 to 1.5 V (vs Li + /Li), Li 5 VS 4 was formed, and a high capacity of ∼750 mAh g −1 was obtained. 16 Lithium-containing metal polysulfide positive electrodes allow using various negative electrodes such as Si, which is an advantage. All-solid-state batteries using Li 2 TiS 3 , Li 3 NbS 4 , and Li 3 CuS 2 have been reported. 10,17,18 However, they showed relatively low capacities of 380-425 mAh g −1 due to the low amount of Li. We focused on Li x VS y with a high Li content (x = 5-9) because of its high theoretical capacity of 626-789 mAh g −1 . Because vanadium sulfides, such as VS 4 and VS 2 , exhibit high electronic conductivity, 16,19 Li x VS y is expected to show high electronic conductivity when Li x VS y contains nano composites of vanadium sulfides. In this study, Li x VS y was synthesized and demonstrated to exhibit a high capacity and high electronic conductivity. Moreover, all-solid-state lithium batteries with Li x VS y have been operated without conductive additives.

Experimental
Preparation and characterization of Li x VS y positive electrodes.-Li x VS y (x = 5-9, y = 4-6) positive electrodes were prepared using mechanochemical treatment techniques. Li 2 S (99%, Kojundo Chemical Lab. Co., Ltd.) and V 2 S 3 (99%; Kojundo Chemical Lab. Co., Ltd.) were mechanically milled at rotation speeds of 300 or 400 rpm for 160 h in a dry Ar atmosphere. X-ray diffraction (XRD) measurements were performed on the prepared Li x VS y positive electrodes using an X-ray diffractometer (Empyrean, Malvern Panalytical Ltd.). To evaluate the electronic conductivity of Li x VS y , pellets of Li x VS y were prepared by pressing the powder at 360 MPa. The pellet was sandwiched between two stainless-steel rods used as current collectors. To evaluate the ionic conductivity of Li x VS y , a three-layered pellet of Li 3 PS 4 glass/Li x VS y /Li 3 PS 4 glass was prepared by pressing the powder at 360 MPa. Li 3 PS 4 glass was prepared by mechanochemical milling of Li 2 S and P 2 S 5 , as described in previous studies. 20 The five-layered pellet of Li-In/Li 3 PS 4 /Li x VS y /Li 3 PS 4 /Li-In was then prepared by attaching a lithium foil (99.9%, 0.2 mm t ; Honjo Metal Co., Ltd.) (φ = 7 mm) and an indium foil (99.999%; 0.1 mm t ; Furuuchi Chemical Corp.) (φ = 8 mm) to both sides of the prepared three-layered pellet. The fivelayered pellet was sandwiched between two stainless-steel rods. DC polarization tests were performed on these pellets using an electrochemical measurement device (Celltest 1470E; Solartron Analytical). For the evaluation of electronic conductivities, 50, 100, and 150 mV were applied to the pellets. For ionic conductivities, 8, 10, 13, and 15 mV were applied.
Construction of all-solid-state batteries with Li x VS y positive electrodes.-Composite positive electrodes were prepared by gentle ball milling of Li x VS y , argyrodite-type sulfide SEs, and acetylene z E-mail: misae-otoyama@aist.go.jp black (AB). All-solid-state lithium cells (10 mm in diameter) were constructed by uniaxial pressing as follows. Argyrodite-type sulfide SE powder (80 mg) was pelletized, and the composite positive electrode powder (5 mg or 10 mg) was placed on the SE pellet and pressed at 360 MPa for 5 min. A lithium foil (φ = 8 mm, 0.2 mm t ) and an indium foil (φ = 9 mm, 0.3 mm t ) were attached to the other side of the SE pellet as a counter electrode and then pressed at 90 MPa for 2 min. The pellet was sandwiched between two stainlesssteel rods and initially charged to 3.6 V (vs Li + /Li) and discharged to the initial charge capacity at a current density of 0.13 mA cm −2 at 25°C using a charge-discharge measuring device (TOSCAT-3100; Toyo System Co., Ltd.). From the 2nd cycle, the cells were charged and discharged with a cut-off voltage of 0.6-4.0 V (vs Li + /Li) and regulation of the initial charge capacity. In the rate performance test, current densities were changed from 0.13 to 0.25, 0.64, 1.3, and 0.13 mA cm −2 every 10 cycles.

Results and Discussion
Preparation and electrochemical properties of Li x VS y positive electrodes.-Li 5 VS 4 , Li 6 VS 4.5 , Li 7 VS 5 , Li 8 VS 5.5 , and Li 9 VS 6 were prepared by ball milling Li 2 S and V 2 S 3 . Figure 1 shows the XRD patterns of the Li x VS y positive electrode. Diffraction patterns of Li 2 S and LiVS 2 were observed in the XRD patterns of Li x VS y , suggesting that Li 2 S reacted with V 2 S 3 and formed LiVS 2 . Some of unreacted Li 2 S also remained. The intensity of the peak attributed to LiVS 2 decreases with increasing x and y in Li x VS y . Therefore, Li x VS y samples are composed of two phases with various ratios of Li 2 S and LiVS 2 corresponding to x and y values in Li x VS y . The prepared samples in the present study are called Li x VS y . The electronic and ionic conductivities of Li x VS y and Li 2 S measured by DC polarization are listed in Table I. Li x VS y showed high electronic conductivity, even though it included Li 2 S which is an insulator. LiVS 2 , prepared by mechanically milling Li 2 S and V 2 S 3 , showed a high electronic conductivity of ca. 10 −1 S cm −1 . One of the reasons for the high electronic conductivity of Li x VS y is that Li x VS y contains LiVS 2 , which exhibits high conductivity. The ionic conductivity of Li x VS y was ∼10 −6 S cm −1 , which was higher than that of Li 2 S (< 10 −8 S cm −1 ). 21 It is suggested that LiVS 2 enhances ionic conductivity of Li x VS y . Figure 2 shows the results of charge-discharge tests of Li x VS y . Figure 2a shows the initial charge-discharge curves of the cells with composite positive electrodes consisting of Li x VS y , argyrodite-type SE, and AB with a weight ratio of 40:50:10%. The cells were charged to 3.6 V (vs Li + /Li) and discharged to the initial charge capacity under a current density of 0.13 mA cm −2 at 25°C. The initial charge capacities, theoretical capacities, and percentages of the initial capacities relative to the theoretical capacities of Li x VS y are listed in Table I. The theoretical capacity is calculated as the capacity when all the Li in Li x VS y reacts. The initial charge capacity of the cell with Li 5 VS 4 was almost equivalent to its theoretical capacity. In contrast, the electrodes with x = 6-9 exhibited ∼80%-90% of the theoretical capacity. With increasing x, the theoretical capacities increase because the Li content in Li x VS y increases. However, they contain a higher amount of Li 2 S, resulting in lower utilization ratio due to insufficient electronic conduction paths. The data of Li 2 S was extracted from the result of initial charge capacity of an all-solid-state cell with a composite positive electrode consisting of Li 2 S, Li 3 PS 4 glass, and vaper grown carbon fiber (VGCF) in a weight ratio of 50:40:10 with a Li 2 S mass of 0.9-1.1 mg cm −2 . 21 Compared with Li 2 S and Li x VS y , Li x VS y (x = 5-9) showed higher conductivity and higher capacity. The cell with Li 8 VS 5.5 showed  the highest initial charge capacity, suggesting that Li 8 VS 5.5 is a suitable ratio of Li 2 S and LiVS 2 for exhibiting high capacity. Figure 2b shows the cycle performance of cells with Li x VS y . The cells were charged and discharged at current densities of 0.13, 0.25, 0.64, and 1.3 mA cm −2 . At the 1st cycle, the cells were charged to 3.6 V (vs Li + /Li) and discharged to the initial charge capacity. From the 2nd cycle, the cells were operated with a cutoff voltage of 0.6-4.0 V (vs Li + /Li) and regulation of the initial charge capacity. Li 5 VS 4 and Li 6 VS 4.5 showed better rate performances because of relatively high electronic conductivity.
In addition, morphology of composites of Li 2 S and LiVS 2 are supposed to effect battery performance. Further morphological studies of Li x VS y will reveal a relationship of battery performance and structures of Li x VS y . During the charge and discharge reactions, it is supposed that delithiation and lithiation occurred in Li 2 S and LiVS 2 . Xu et al. reported that composite cathodes of S and VS 2 changed to Li 2 S and LiVS 2 , respectively, after lithiation. 22 LiVS 2 has a layered structure, which changes to VS 2 after delithiation. 23 Takada et al. reported that sulfide all-solid-state cells with LiVS 2 were charged and discharged reversibly. 24 It is suggested that LiVS 2 enhances the cycle and rate performances of Li x VS y , as shown in the results for Li 5 VS 4 and Li 6 VS 4.5 , which contain higher amounts of LiVS 2 .
Construction of all-solid-state cells without conductive additives.-Owing to the high electronic conductivity of Li x VS y , allsolid-state cells with Li x VS y may not require conductive additives. We focused on Li 5 VS 4 exhibiting high electronic conductivity and the highest utilization ratio. Composite positive electrodes were prepared by mixing Li 5 VS 4 and argyrodite-type SEs with compositions z = 100, 90, 85, 80, and 70 in z:(100−z) (wt%). Figure 3a shows the initial charge-discharge curves of the cells with z = 100-70. The cells, except for z = 100, were charged to 3.6 V (vs Li + /Li) and discharged to the initial charge capacity. The cell with z = 100 was discharged to 0.6 V (vs Li + /Li). The initial charge capacities at z = 100, 90, 85, 80, and 70 were 48, argyrodite-type SE in z:(100−z) (wt%). The cells were initially charged to 3.6 V (vs Li + /Li) and discharged to the initial charge capacities under a current density of 0.13 mA cm −2 at 25°C. The cell with z = 100 was discharged to 0.6 V (vs Li + /Li). From the 2nd cycles, the cells were charged and discharged to the initial charge capacity or cut-off voltage of 0.6-4.0 V (vs Li + /Li). 260, 280, 586, and 574 mAh g −1 , respectively. The cell with z = 80 exhibited the highest charge capacity, which was 94% of the theoretical capacity of Li 5 VS 4 . It was assumed that the composition of z = 80 formed better electronic and ionic conduction path. Figure 3b shows cycle performance of the cells until 10 cycles. The cells with z = 90, 85, 80 and 70 exhibit no capacity degradation. It was demonstrated that all-solid-state cells with Li x VS y showed high capacity and cycle stability without conductive additives because of the high electronic conductivity of Li x VS y .

Conclusions
Li x VS y positive electrodes were prepared via the mechanochemical treatment of Li 2 S and V 2 S 3 . The XRD patterns of Li x VS y revealed that it was composed of Li 2 S and LiVS 2 . Since the electronic conductivity of LiVS 2 is ∼10 −1 S cm −1 , Li x VS y shows a high electronic conductivity of ∼10 −1 -10 −2 S cm −1 , although it includes insulating Li 2 S. The all-solid-state cells with Li x VS y showed a high capacity of 588-649 mAh g −1 . A cell with composite positive electrodes with Li 5 VS 4 and SEs at 80:20 (wt%) particularly showed 94% of the theoretical capacity without conductive additives. Hence, Li x VS y is a promising positive electrode for the high energy density of all-solid-state batteries because it shows a high capacity with no conductive additives and a high loading of Li x VS y . The detailed structure and reaction mechanism of Li x VS y will be revealed in the near future.