LITHIUM ION BATTERIES

Transcription

1 LITHIUM ION BATTERIES 1

2 Electrodes & Jelly roll 2

3 3 Types of Lithium ion batteries 원형, 원통형, cylindrical 각형, prismatic 폴리머, polymer (pouch type)

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5 Materials composing electrodes 5

6 6 Terminology-1 <Terminology: Materials> 1. Material 2. Electrode 3. Cell Electrode materials : active material, positive(negative) E. material, cathode(anode) material (frequently misused) Positive electrode material (Cathode material) : LCO (LiCoO 2 ), NCMxyz (Li(Ni x,co y,mn z )O 2 ), LMO (LiMn 2 O 4 ), LFP/LMP (Li(Fe or Mn)PO 4 ) (layer-structured, spinel, olivine structure) Negative electrode material (Anode material) : graphite, Si (Si-C composite, SiOx, Si-alloy ) Binder : adhesive to fix active materials on current collector : PVDF(NMP), SBR/CMC(water) Electronic conducting agent : improves electronic conductivity in electrode : fine carbon powder Electrolyte additive : additives in electrolyte solution to improve properties

7 7 conducting agent active materials binding materials metal foil

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9 9 Terminology-2 1. Material 2. Electrode 3. Cell <Terminology: Electrode> Loading level : mass of electrode material per unit area : this is related to thickness of electrode Specific capacity (or gravimetric capacity) : capacity (current/power) per mass (mah/g, Ah/kg, Wh/kg ) Volumetric capacity : capacity per volume (mah/cc, Wh/L ) Electrode density : density of electrode : determined by density of material, porosity of electrode : important factor determining volumetric capacity N/P ratio : ratio of total capacity of anode/cathode (typically >1) SEI (solid electrolyte interface) layer : a kind of coating layer on the electrode which is formed by side reactions between electrode and electrolyte : property of SEI is a key factor for battery performance

10 10 Terminology-3 1. Material 2. Electrode 3. Cell <Terminology: Cell-1> Reversible capacity : the capacity usable repeatedly (initial CH capacity loss) Irreversible capacity : capacity loss during (1 st ) cycle : loss related to formation of SEI layer, side reactions, partial structural degradation of active material, etc. (Coulombic) Efficiency : 1 st discharge capacity / 1 st charge capacity : high irreversible capacity-low efficiency * A battery cell should be designed considering irreversible capacity of cathode and anode. balance of efficiency C-rate : term related to charge/discharge speed : determined by total capacity and charge/discharge current ex) 1C: 200mA current for 200mAh capacity (it takes 1 h) 0.2C: 40mA current for 200mAh capacity (it takes 5h)

11 11 Terminology-4 <Terminology: Cell-2> 1. Material Rate performance : a measure of kinetics (power) ex) 1C-capacity/0.1-C capacity : important factor for high speed charge/discharge 2. Electrode 3. Cell-2 Capacity retention (Cyclability, Longevity ) : change of discharge capacity with repetition ex) 80% after 300cycles Safety : ignition, swelling : heat fuel oxygen : (Joul heating, short circuit) (flammable solvents) (decomposition of cathode material) : DSC measurement

12 Background 12 Where does thermal runaway take place? Internal short & spark Cathode Ignition by flame (induced by internal short) Anode Heat transfer Heat transfer Separator melting & shrinkage Regional flame & shot area expansion O 2 Flammable gas from - Oxidation of elect. O2 release from AM Oxidation Rxn. at gas phase of elect. Active radical formation Flammable gas from - thermal degradation of electrolyte Flammable Gas from - Thermal degradation of SEI - Reduction of elect. - Reaction with Lithium(Li/Li x C 6 ) Thermal shrinkage & melt-down of separator expand short area Thermo-mechanically stable separator is required Flammable electrolyte act as fuel for combustion Non-flammable electrolyte(nfl) is required

13 13 FIG : 율속 (C-rate) 에따른방전곡선의변화 (NCA, LMO). (C-rate = 0.1C, 1C, 2C, 5C) C-rate 의증가에따라용량및방전전압감소. 결정구조적으로 3 차원리튬이동 channel 을갖는 LMO 의경우율특성이우수함

14 14 Polarization & Overpotential 1. Definition 2. Mechanisms of polarization Polarization : Electrochemical reaction is composed of several steps such as diffusion of mass, charge transfer, precipitation or crystallization, and so on : retardation of a step may delay the overall reaction : this is called polarization Overpotential : because of the polarization, the reaction becomes slow or even stops : to maintain the reaction, additional increase of applied electric potential is necessary : overpotential or overvoltage Factors causing overpotential - Resistance overpotential (η R ): film resistance - Concentration overpotential (η C ): depletion of reactants - Activation overpotential (η A ): large activation energy

15 15 Resistance overpotential : formation of some layer whose electric resistance is large : this causes retardation of an electrochemical reaction : ex) a film layer deposited on the electrode of lithium ion battery during chargingdischarging process by decomposition of electrolyte solvents : cf) additive reagents which forms conductive film Concentration overpoteitnal (diffusion overpotential) : when diffusion of reactants to the electrode is the rate-determining step : very fast charge transfer, or very slow diffusion of reactants in the solution : cf) stirring of electrolyte solution, or raising temperature to increase diffusion speed Activation overpotential : when a step requires large activation energy