Steve Harris. UC Berkeley

Transcription

1 Li Ion Batteries Steve Harris General MotorsR&D UC Berkeley 1

2 Outline Li battery basics Samples from my research What are the problems? Whither h Li batteries? 2

3 Some Basics AB + C A + BC G 3

4 Some Basics AB + C A + BC AB + C A + B + + e - +C A+BC G = -nfe B Li A, C electrodes Electrolyte 4

5 Power Supply Li or graphite LiCoO 2 When Li is part of a strongly bonded system, free energy is low so voltage is high: positive electrode ( cathode ) When Li is part of a weakly bonded system, free energy is high so voltage is low: negative electrode ( anode ) 5

6 Negative Electrodes Li metal Lowest possible potential Reproducible chemistry High mass and volume density 6

7 The (Dreaded) Solid Electrolyte Interphase (SEI) 7

8 The (Dreaded) Solid Electrolyte Interphase (SEI) e e e e e 8

9 The (Dreaded) Solid Electrolyte Interphase (SEI) e e e e e e LiF LiOH Li 2 CO 3 9

10 The (Dreaded) Solid Electrolyte Interphase (SEI) e e e e e e e e e LiF LiOH Li 2 CO 3 10

11 The Solid Electrolyte Interphase (SEI) 10% (10% Li can t get through 11

12 Goal: Understand d Degradation Fundamentals Hypotheses: Battery failure related to failure in Li transport Battery yfailure begins at a flaw or an inhomogeneity Conclusion: Present models assume homogeneous materials (Newman) Little data on microscopic inhomogeneities Study Li transport at the microscale How do we visualize Li transport? Is it homogeneous?

13 Li Transport at themicroscale Between Particles Within particles Across the SEI My experiments aim to learn about each of these issues

14 Li Transport at themicroscale Between Particles Within particles Across the SEI My experiments aim to learn about each of these issues

15 How Do We Visualize Li Transport? 15

16 Are Electrodes Homogeneous? 2 mm A homogeneous model will not comprehend 16

17 X Ray Nano CT with N. Brandon and P. Shearing, Imperial College; D. Kehrwald, GM Composite graphite electrode from Lishen Sample volume, 43 x 348 x 478 μm Voxel size = 480 nm

18 Homogeneity in Tortuosity Charge balance Mass balance Is tortuosity homogeneous? If not, does it matter?

19 Homogeneity in Tortuosity µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm µm m µ z y µm µm µm µm µm FE Calculation

20 Homogeneity in Tortuosity Garcia, Purdue Por rosity [ ] 0,8 0,7 0,6 0,5 04 0,4 0,3 0,2 01 0,1 Sub Sample porosity Porosity geometry 1 Sub sample tortuosity Tortuosity geometry Sub sample no. [ ] Could get faster safe charging rates Could go to higher states of charge Tor rtuosity

21 Solid Electrolyte Interphase (SEI) ~ 20 nm E lectrode Formed from electrolyte decomposition Conducts Li +, blocks electron transport SEI quality influences internal resistance, capacity, and safety SEI degradation is one of the commonest causes of failure

22 Transport in the SEI A large literature on SEI, practically all concentrating on Formation mechanisms Composition Impedance measurements How does SEI function as transport medium? Li + moves Via porosity Via grain boundaries between SEI components (E. Peled) Via interstitials and vacancies (J. Newman) As though SEI were a polar ionic liquid (G. Smith) Almost no direct experimental information Almost no direct experimental information About Li + transport About SEI electrical conductivity

23 Time of Flight Secondary Ion Mass Spectrometry Primary Ion Beam (Au +,C 60+ ) Secondary Species (+/, e, neutrals )

24 Lithium Transport in the SEI: Strategy Cu Electrode Cu First SEI Layer Soak in Depth Profile Electrolyte DMC Rinse 7 LiClO 6 4 LiBF 4 DMC Rinse Cu Cu Cu 1. Cu Electrode (quartered) in coin ½ cells: Cu electrode isolates transport within SEI (no driving force for insertion) 2. Create an SEI with natural abundance electrolyte, 7 LiClO 4 ( 6 Li: 7 Li ~ 0.08) 3. Soak SEI 3 films with isotopically labeled electrolyte, 6 LiBF 4 ( 6 Li: 7 Li ~ 20) BF ti 4 and 6 Li + are tracers for ion transport in the 7 LiClO 4 SEI 4. Depth profile with TOF SIMS

25 BF 4 or B + Depth Profile Electrolyte Copper Normalized In ntensity (a.u.) 30s 3min 15min Depth (nm) Electrolyte diffuses through pores in SEI BF 4 cannot move without Li +

26 Presence of a dense blocking layer sity (a.u.) Norma alized Inten 11 B + 7 Li 2 CO Depth (nm) Top 5 5 nm: porous layer into which h LiBF 4 can diffuse Below: densely packed layer, hard for LiBF 4 to go through

27 Transport of 6 Li + (electrolyte) in SEI Yue Qi, GM Li/ 7 Li s 30s 3min 15min Natural Abundance: ~ Depth (nm) What is the rate; dependence on: T, V, SEI thickness/composition/morphology

28 Now What? The problems Safety Cost Price of entry Need 4x price reduction Durability But durability and cost are interchangeable Low Energy Density Need 4x range Today s approach Chemistry changes in Li ion ion batteries towards +50% improvement Revolutionary changes 28

29 Let s Redesign Every Component in the Cell Current collector No more Cu (too heavy, too scarce) Use Al on the negative side with organic electrode (sugar) at 0.6V Binder Conducting, so no more conductive carbon Active material Layered particles Designed shape Guoying Chen, LBL Solvent Works at low temperatures

30 Let s Redesign Every Component in the Cell Separators Can we block dendrites? Can we capture bad guys like HF? No more in situ SEI Al 2 O 3 ALD coatings (enable nanoparticles?) No limit on charging rate Nanostructured anode with artificial SEI High interparticle transport rates New cell architectures But very non uniform tortuosity Self assembly

31 Closing Notes Almost every vehicle will be electrified (CAFÉ) Cost vs gasoline will drive the extent of electrification i Batteries are 50% materials, 50% manufacturing Packs are 50% batteries, 50% controls, cooling Electrification model Mild hybrid (Buick Regal e Assist) Strong hybrid (Toyota Prius, parallel) Extended range EV (Chevy Volt, series) Pure EV (Nissan Leaf) Better Place (Israel) Other? 31

32 Shrinking Core Model A core of 1 phase is surrounded by a shell of a second phase Phase boundary movement is driven by transport of Li C 6 Li 0.8 C 6 LiC 6 Assumptions: 1. Particles are small enough so that environment is isotropic 2. Particles are homogeneous and isotropic 3. Transport is by bulk diffusion

33 Optical Cell Zoom in ADVANTAGES 1. Face up electrodes let us use graphite s color changes to measure in-situ spatial profiles. 2. The side-by-side arrangement large concentration gradients. While undesirable in a commercial cell, large gradients are useful when we want to learn about transport. 3. Convert (a) Li transport perpendicular to the current collector through 0.1 mm, to (b) Li transport parallel to the current collector through < 1 cm, emphasizing transport

34 3D Calculations (Garcia) Separator Li Li

35 Presence of Li 2 O/Li 2 CO 3 Layer 11 B + 7 Li OCO 3 Cl Surface 10 m 10 m 10 m 11 B + 7 Li OCO 3 Cl 6 nm 10 m 10 m 10 m SIMS maps consistent with depth profile results Observation of 35 Cl indicates we are within the SEI