Transportation sector : Understanding the scale • Role of transportation and electricity in CO2 emissions1,2 • 14% of total CO2 emissions (6 GtonCO2/year) • Transportation : • Electricity and heat : • 25% of total CO2 emissions (11 GtonCO2/year) • Number of vehicles will triple by 2050 • 1Kromer & Heywood, MIT, 2007 • 1 2IEA, World Energy Outlook 2012
The challenge for energy storage : High energy density and high power density at low cost • Cars run 1-4 miles per kWh (35 Mpg => 0.25kWh/mile) • 200 mile driving range requires 50-200kWh • Car operation requires power of 100-200hp (75-150kW) • Volumetric energy density for different storage technologies • Current Li-ion batteries have ~15% the volumetric energy density of gasoline • At predicted ~500$/kWh, Li-ion batteries for 200 mile range at 0.25kWh/mile costs about 25 000$ ! 3Cell level, 80% charge to wheel efficiency (data from Samsung 403450 LiCoO2 cell and LG chem) 1 9.7 kWh/l for C8H18 , 20% engine to wheel 22.5 kWh/l for CH4 at 250 bars, 40% engine to wheel
Requirements for rechargeable batteries • Energy Density (kWh/l) • Energy per unit volume, must be high for long driving range • Energy/volume = Energy/charge x #Charge/mass x #mass/volume • Mass density (kg/L) • Voltage (V) • Power Density (kW/l) • Charge capacity (Ah/kg) • Energy per unit time and volume, must be high for fastacceleration and fast charge • Largely determined by the diffusivity of Li in the cathode and anode • Safety • Temperature the battery can sustain without catching fire • Cycle life • Number of times the battery can be charged/discharged without energy loss
Li is shuffled from a high energy state to a low energy state (discharge) and vice-versa for charge Li-ion batteries : How they work • Low voltage means high energy, high voltage means low energy • Anode (Low voltage) • Li is in a high energy state • Cathode (High voltage) • Li is in a low energy state • Electrolyte • Blocks e- passage, let Li+ pass • (Low energy) • (High energy) • (Low voltage) • (High voltage) • Discharge : Li goes from the anode (high E) to the cathode (low E) • Work is extracted (run your car) • Charge : Li goes from the cathode (low E) to the anode(high E) • Work must be provided (plug the battery into the wall)
Li is shuffled from a high energy state to a low energy state during discharge, vice-versa for charge Li-ion batteries : How they work • Charge1 • Discharge1 • Anode and cathode : Host materials that can intercalate Li in interstitial sites • Very little changes to the crystal structure as Li is inserted or removed => Reversibility • Reversibility => Rechargeability 1Pictures inspired by “Park, Goodenough. JACS, 2013, 135” & http://www.clker.com/clipart-14374.html
Li-ion batteries components : The electrolyte1 • Lithium salts (LiPF6) dissolved in organic solvents (EC:DMC) • Electrolyte bandgap blocks electron passage1 • EC:DMC solvent • LiPF6 salt dissolved in organic liquid EC:DMC (ethylene carbone – dimethyl carbonate) • Electronic energy gap forbids electron passage (only Li+ can pass through) • Anode and cathode voltages must be within the electrolyte stability window (2-4.5V vs Li/Li+). • Upper limit on voltage => Upper limit on energy density • Flammable at high temperature 1Goodenough, J.B & Park, K.S. “The Li-ion battery : A Perspective”, JACS, 135, 2013, 1167-1176.
Safety : Fires caused by Electrolyte and Cathode • Many cases of batteries catching fire (laptops, cars, airplanes) • Malfunction (ex:short-circuit) => Temperature Rise => Oyxgen evolution at the cathode => Combustion of the electrolyte • Cathode (ex:LiCoO2) contains oxygen. Above a certain temperature, the cathode material starts decomposing and O2 is emitted. • The electrolyte is an organic solvent, i.e. a fuel. • Live demonstration (http://www.youtube.com/watch?v=pizFsY0yjss)
Li-ion cathode (Low Li Energy, High Voltage) • 3 cathode materials have been commercialized : LiCoO2, LiMn2O4, LiFePO4 • Each have their advantages and drawbacks : No silver bullet • LiCoO2 • Most widely used cathode materials (cell phones, laptops, Tesla) • High energy density (170mAh/g , 3.9V vs Limetal) • Charge time [high power equipment] ~ 1 hour (C-rate~1C)1 • Unsafe (O2evolution temperature <200oC) • LiMn2O4 • Used in Chevrolet Volt and Nissan Leaf • Lower energy density than LiCoO2(110mAh/g , 4V vs Limetal) • Charge time [high power equipment] is ~3 min (C-rate~20C)1 • Safer than LiCoO2 (O2 evolution temperature ~ 300oC) • Lifetime concerns (Mn dissolution, engineering required for higher lifetime) • 1In practice, this rate can only be achieved at very high currents, i.e. in a high power charging station
Li-ion cathode (Low Li Energy, High Voltage) • 3 cathode materials have been commercialized : LiCoO2, LiMn2O4, LiFePO4 • Each have their advantages and drawbacks : No silver bullet • LiFePO4 • Only used in niche applications (Power Tools, Electrical Buses, Motorcycles) • Lower energy density than LiCoO2 and LiMn2O4(170mAh/g , 3.4V vs Li metal), due to low voltage and nano-processing • Very high power density : Charge [high power limit] is 1 minute1 (C-rate=50C) • Very safe (O2 evolution temperature ~ 500oC) • Most expensive cathode due to nano-processing • 1Can only be achieved at very high currents
Li-ion battery anodes (High Li Energy, Low Voltage) • Li metal : Ideal anode but unsafe • Lithium metal is the highest possible energy state for Li and hence the lowest possible voltage (0V vs Li metal) • High charge capacity (3600 mAh/g) • Unsafe upon battery operation: Dendrites form and short-circuit the battery • Commercial anode : Graphite (LiC6) • Lithium intercalates between graphene sheets (LixC6, 0≤x≤1) • Low voltage : 0.2V vs Li metal • Voltage is below the stability limit of the electrolyte, but surface passivation (SEI) occurs during battery operation • Li • Co • High charge capacity : 372 mAh/g • (but low compared to 3600 mAh/g for Li metal)
Energy Density and Power Density • High energy density with respect to other rechargeable batteries • Low energy density with respect to IC engine and fuel cells • Power density comparable to IC Engine • Compared to other rechargeable batteries1 • Compared to IC engine and fuel cells2 1Tarascon, J.M. & Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature414, 359–367 (2001). 2Srinavasan, V. Batteries for vehicular applications, AIP conference proceedings, 1044, 283 (2008).
Electric cars on the market • Hybrid Electric Vehicles (HEV) : Toyota Prius, 24k$-29k$ 1 • Toyota Prius • Electrical motor (EM) assists internal combustion engine (ICE) • Battery is charged by the engine (not really an electric car) • EM+ICE is more efficient than ICE alone (acceleration assistance, regenerative breaking, start and drive in electric mode, smaller and more efficient engine) • 500Wh of electric energy used (below 500$ at current Li-ion battery cost) • Toyota Prius (HEV) vs Lincoln MKZ (ICE) : 41Mpg (city) vs 18 Mpg (city), same cost1 • 1Ceder G. 3.70 : Materials Science of Clean Energy, Lecture Notes, MIT, 2013.
Electric cars on the market • Plug-In Hybrid Electric Vehicles (PHEV) : Chevrolet Volt, 39k$ 1 • Electric drive train, LiMn2O4 battery • First 40 miles2 from the battery, rest from the electricity generated by an ICE • About 70% of all daily driving in the US is less than 40 miles • Battery guaranteed 8 years or • 100 000 miles, with 10-30% battery energy loss after 8 years. • 1Prices do not include tax incentives • 2EPA certified driving range • 3Wikipedia
Electric cars on the market • Electric vehicles (EV) : Nissan Leaf (35k$), Tesla Model S (60-80k$)1 • Electric drive train powered only by a battery • Nissan : 75 miles2 (24kWh battery), 35k$ 1 • Tesla Model S : 160-265 miles2 (40-85kWh battery), 60-80k$ 1 • Tesla model S • Nissan Leaf • 1Prices do not include tax incentives • 2EPA-certified range
Costs compared to an average 23 mpg car1,2 • PHEV : 16 000$ extra , EV : 17 000$ extra (without tax incentives) • Gasoline : 0.50$ per kWh (3.7$/gallon) , Electricity : 0.15$ per kWh • Average fuel cost (3.7$/gallon, 13 000 miles a year, 23mpg) : 2100$/year • Fuel savings over 5 years : 35mpg car 4000$, PHEV 7250$, EV 9000$ • After 5 years : Price of ICE, PHEV and EV are similar (with tax incentives) 1 Mackenzie, D. 2.65 Lecture Notes, Massachusetts Institute of Technology. 2www.fueleconomy.gov
CO2 emissions : Better than ICE but depends on electricity mix • Different studies predict different results (car efficiency, electricity mix, driving habits, etc) • Kromer & Welwood, MIT, 2007 • Karplus, MIT, 2012 • All technologies • Nissan Versa (35mpg) : 317 g CO2/mile • Chevrolet Volt (PHEV) : 260 g CO2/mile (US), 200 g CO2/mile (Mass) • Less CO2 • Nissan Leaf : 230 g CO2/mile (US), 140 g CO2/mile (Mass) • Toyota PRIUS (HEV) : 222 g CO2/mile • (www.fueleconomy.gov, EPA standards)
CO2 emissions depend on the electricity mix • Left side of error bar : All natural gas, Right side of error bar : All coal1 • 1 Kromer & Heywood, MIT, 2007 • Accepted trends • US electricity mix : 42% coal, 25% natural gas, 19% nuclear, 13% hydro + renewables • Carbon-free electricity : PHEVs displace ~70% of CO2 emissions, EV displace 100% • All coal : HEV is better than PHEV, but PHEV still better than best ICE • All natural gas : ~50% of CO2 reduction with PHEV (better than HEV and ICE)
Cost projections for battery price (pack level) • Cost reduction is slowing down • Cost projections (pack level) • Cell level • Battery pack cost today : 650$-1000$/kWh (32 500$ - 50 000$ for 200 miles) • Foreseeable cost with current Li-ion battery technology : ~400-500$/kWh • Even then : Full EV battery cost ~20 000-25 000$ for a small car with 200 miles • Major cost decreases required for EV adoption (via increase in energy density)
Prospects for future batteries • Li-air battery Pictures courtesy: G. Girishkumar et al., JPCL (2010) metal anode air cathode 2Li+ + 2e− + O2 Li2O2 Li2O2 2Li+ + 2e− + O2 Veq: 2.96 V • High energy density only possible if O2 is compressed, engineering challenges • [1.4-1.9 kWh/l if O2 is compressed 0.6-0.7 kWh/l if O2 is not compressed] • Low roundtrip efficiency (50-60%), Poor cyclability (10-100cycles) • Other issues (Li metal anode, etc), R&D required • Other R & D : Li-sulfur, Conversion materials, Multivalent, etc.
Conclusions • Li-ion batteries : Reversible shuffling of Li between anode and cathode • Different anode and cathode materials have advantages and drawbacks (cost, energy density, safety, cyclability) • HEV’s have penetrated the market and reduce CO2 emissions • PHEV’s make sense economically, but little market penetration. • Li-ion batteries are too costly for full EV’s. Need new materials with higher energy density.