Fundamental Study on Li Metal Dissolution and Deposition on Cu Foil in Nonaqueous Electrolytes with 3DOM Separator

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1 217 BLI X, Symposium on Energy Storage, June 27-29, 217, at IBM- Research Almaden in San Jose, CA, USA Fundamental Study on Li Metal Dissolution and Deposition on Cu Foil in Nonaqueous Electrolytes with 3DOM Separator Kiyoshi Kanamura, Naohiro Kobori, and Hirokazu Munakata Department of Applied Chemistry, Graduate School of Urban Environmental Sciences Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo, Japan address:

2 体積容量密度 (mah dm -3 ) Energy density per volume (ma h dm -3 ) Introduction : Lithium Secondary Battery Lithium secondary batteries are needed to realize high energy density Li 2 Pb 5 Li 3 Sb Li 3 As LiAl Li 22 Sn 5 LiC 6 炭素系 Carbon anode Li 22 Si 5 Alloy 合金系 anode Energy 重量容量密度 density per (mah weight g -1 ) (ma h g -1 ) 3861 ma h g -1 Li Li metal リチウム金属 anode < In the future > Rechargeable batteries 5 W h kg -1 Lithium metal has high capacity density (3861 ma h g -1, V vs. SHE) as excellent anode material. Li metal is the ideal anode material for lithium secondary batteries.

3 Introduction : Problem of Lithium Secondary Battery Li metal cannot be used in practical batteries due to low cycleability and safety problem, which are related to the morphology of Li metal deposited during charging process. In order to use Li metal as the anode in rechargeable batteries, the Li dendrite formation has to be suppressed.

4 Introduction : Lithium Dendrite Low High Current density Formation of SEI Flat and smooth Uniform Non-uniform Dendritic growth The formation of lithium dendrite is related to non-uniform current distribution on a lithium metal anode in the course of the charging process of battery. SEM image of Li dendrite

5 Behavior of Surface Film on Li metal In Electrolyte Before immersion in electrolyte After immersion in electrolyte In Electrolyte Surface film is changed by chemical reaction between electrolyte or impurities and native surface film.

6 Surface Structure of Li Metal & Morphology Surface film formed in electrolyte is not so uniform. Concentration of Current flow : Large current distribution during Li metal deposition 1M PF/EC/DEC 2 ma cm -2 1M PF/EC/DEC.5 ma cm -2 High Resistivity Surface Layer Low Resistivity 1M PF/EC/PC 2 ma cm -2.2M PF/EC/DEC.5 ma cm -2 : Li + ion (Current) flow Li Metal The morphology of Li metal depends on current, concentration of slat and kind of solvent. The surface structure of Li metal is determined by chemical reaction between electrolyte components and native surface film.

7 Control for Surface Film with HF Additive LiF, Li 2 CO 3, LiOH Li 2 O, LiOH Li Metal With HF XPS spectra of Li metal surface in propylene carbonate containing 1 mol dm -3 LiClO 4 with HF additive. Without HF LiF, Li 2 CO 3, LiOH, LiCl Li 2 O, LiOH Li Metal XPS spectra of Li metal surface in propylene carbonate containing 1 mol dm -3 LiClO 4.

8 Dynamic Behavior During Discharge and Charge Mass change during lithium metal deposition on Ni substrate in propylene carbonate containing 1 mol dm -3 LiClO 4 with and without HF additive, which are measured with EQCM. Li metal react with electrolyte during dissolution process. 3 mc 2 mc discharge AFM images for Li metal surface deposited in propylene carbonate /1. mol dm -3 LiClO 4 with HF additive. 6 mc discharge 11 mc discharge Figure 12 AFM i mages for Li metal during deposition and dissolution processes under galvanostatic conditions at.2 ma cm -2, (a) 3 mc deposition, (b) 2 mc dissolution, (c) 6 mc dissolution, and (d) 11 mc dissolution. Cylindrical Shape Electrolyte penetration LiPF 6 EC:DEC(=1:1), 2 ma/cm- 2

9 Interfacial Current Distribution Control by Separator Attached to Li Metal Conventional separator Li + Porosity (about 3 ~ 4 %) Columnar pore 1mm Three-dimensionally ordered macroporous (3DOM) polyimide (PI) separator Li + Porosity (about 7 %) Porous structure Constant current density 1mm 9/19

10 Preparation of 3DOM Separator Three Dimensionally Ordered Macroporous (3DOM) structure Polyimide Top side (SEM) Base side 3DOM separator provides uniform current distribution to Li metal surface.

11 Properties of 3DOM Separator 3DOM PI separator has high affinity to electrolyte solutions due to high hydrophilicity. Electrolyte solution 3DOM PI separator Conventional PP separator 1 mol dm -3 LiPF 6 in EC : DEC = 1 : 1 (in vol.) 8. mm 4. mm 1 mol dm -3 LiPF 6 in EC 7. mm x *Ref.: 3DOM J.-R. PI separator Lee et al. Journal has high of Power affinity Sources to electrolyte 216 (212) solutions due to high hydrophilicity.

12 Symmetrical Cell Li/Li (Utilization 3 %, 8 ma h, 16 ma) 4 1cycle 4 1cycle 2 2 Voltage / mv -2 Voltage / mv Time / hour Time / hour 4 2cycle 4 3cycle Voltage / mv 2-2 Voltage / mv Time / hour Time / hour

13 SEM Images of Li metal During 1 st Cycle Li/Cu cell Before the 1 st cycle After deposition After dissolution EC EC : DEC = 1 : 1 EC : DMC = 1 : 1

14 EISs of Li metal During 1 st Cycle EC EC : DEC = 1 : 1 EC : DMC = 1 : Z" / Ohm 6 4 Z" / Ohm 6 4 Z" / Ohm 6 4 Before the 1 st cycle Z' / Ohm Z' / Ohm Z' / Ohm Z" / Ohm 1 5 Z " / Ohm 3 2 Z" / Ohm 3 2 After deposition Z' / Ohm Z ' / Ohm Z' / Ohm Z" / Ohm Z " / Ohm Z" / Ohm After dissolution Z' / Ohm Z ' / Ohm Z' / Ohm

15 Comparison of Separators 3DOM separator PP separator (a) (b) (a) (b) (c) (d) (c) (d) (e) (e) SEM images of lithium metal anode after 1 st charge with 3DOM polyimide separator (a)ec (b)ec : DMC = 7 : 3 (c) EC : DMC = 1 : 2, (d) EC : DEC = 1 : 2 (e) PC SEM images of lithium metal anode after 1 st charge with polypropylene separator (a) EC (b) EC : DMC = 7: 3 (c) EC : DMC = 1 : 2, (d) EC : DEC = 1 : 2 (e) PC

16 Coulombic Efficiency EC EC : DEC = 1 : 1 EC : DMC = 1 : Voltage / V Voltage / V Voltage / V Time / Hr Time / Hr Coulombic efficiency (%) Time / Hr EC EC : DEC = 1 : 1 EC : DMC = 1 : 1 1 st cycle nd cycle rd cycle th cycle th cycle

17 Full Cell Performance Cu : anode & current collector (18 mm) NMC : cathode 3DOM-PI : separator Electrolyte : EC, EC+EMC (3:7), EC+DMC (1:2), EC+DMC (1:1), EC+DMC (1:1), PC, EC+DEC (1:2), EC+PC (1:1) / 1. mol dm -3 LiPF 6 Laminated Cell : 3 cm 4 cm Composite Electrode : NCM 92 %, AB 4 %, PVdF 4 % Thickness : 73 ± 2 mm Density : 2.8 ±.5 g cm -3 Al current collector

18 Test Cells Cathode : NCM (Celion L-113):92 %, AB (acetylene black) : 4 %, PVdF (KF Polymer) : 4 %, Thickness : 73±2 mm (Density:2.8±.5 g cm -3 ) Anode : Cu foil (thickness : 18 mm, weight : 22±6 mg) Separator : 3DOM PI (5±3 mm) Electrolyte (32 ml) : EC, EC/EMC=3/7, EC/DMC=1/2, EC/DMC=1/1, EC/DMC=9/1, EC/DEC=1/2, EC/PC=1/1, PC Preparation of laminate cell Conditions of charge-discharge and rate performance tests voltage : upper limit : 4.2V, lower limit : 2.V, Current : 1-3 cycles ;.1 C, 4-6 cycles ;.2 C, 7-9 cycles ;.5 C Cyclic performance voltage : upper limit ; 4.2 V, lower limit ; 2. V, Current : 1-1 cycles ;.1 C (3.6±.3 ma)

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20 Cycleability of Cells

21 Cycleability of Cells

22 Cycleability of Cells

23 Li Metal Battery with 4 W h kg -1 Cathode: High capacity NMC cathode 17 ma h g ma h g -1 Anode: Li metal/cu (6 mm thickness) foil Electrolyte: EC based electrolyte (EC or EC/DMC) Separator: 3DOM separator Similar to LIB, but different. Energy density estimated from basic research: more than 4 W h kg -1 (near to 1 W h L -1 ) Li metal (2 mm) / Cu foil (utilization 4 ~ 6 %) 3DOM separator NMC (8 mm)

24 Summary for Li Metal Anode 3DOM separator suppress the dendrite formation of lithium metal. The suppression of dendrite formation depends on a kind of electrolyte. EC provides the best performance. When using extra amount of Li metal, cycleability of Li metal is improved very much. The utilization of Li metal anode is important. The full cell with NCM cathode and Cu anode can be cycled. However, the rechargeability of these cells is not so good depending on the kind of electrolyte. In this case, EC electrolyte exhibited the best performance. What is a criteria for choice of electrolyte? Solvent + LiPF6 HF or Other F compounds

25 Acknowledgement This work has been done by financial supports from NEDO, JST (ALCA-spring) and 3DOM Inc.. Dr. Nishikawa provides useful SEM observation results.